diff --git a/gopls/internal/lsp/cache/session.go b/gopls/internal/lsp/cache/session.go
index 3b0958e..51c8bb7 100644
--- a/gopls/internal/lsp/cache/session.go
+++ b/gopls/internal/lsp/cache/session.go
@@ -581,7 +581,6 @@
 // the directory.
 func (s *Session) ExpandModificationsToDirectories(ctx context.Context, changes []source.FileModification) []source.FileModification {
 	var snapshots []*snapshot
-
 	s.viewMu.Lock()
 	for _, v := range s.views {
 		snapshot, release := v.getSnapshot()
diff --git a/gopls/internal/lsp/cache/snapshot.go b/gopls/internal/lsp/cache/snapshot.go
index 5a15641..80de11a 100644
--- a/gopls/internal/lsp/cache/snapshot.go
+++ b/gopls/internal/lsp/cache/snapshot.go
@@ -917,7 +917,7 @@
 		dirName := dir.Filename()
 
 		// If the directory is within the view's folder, we're already watching
-		// it with the pattern above.
+		// it with the first pattern above.
 		if source.InDir(s.view.folder.Filename(), dirName) {
 			continue
 		}
@@ -1654,16 +1654,18 @@
 // Most likely, each call site of inVendor needs to be reconsidered to
 // understand and correctly implement the desired behavior.
 func inVendor(uri span.URI) bool {
-	if !strings.Contains(string(uri), "/vendor/") {
-		return false
-	}
-	// Only packages in _subdirectories_ of /vendor/ are considered vendored
+	_, after, found := cut(string(uri), "/vendor/")
+	// Only subdirectories of /vendor/ are considered vendored
 	// (/vendor/a/foo.go is vendored, /vendor/foo.go is not).
-	split := strings.Split(string(uri), "/vendor/")
-	if len(split) < 2 {
-		return false
+	return found && strings.Contains(after, "/")
+}
+
+// TODO(adonovan): replace with strings.Cut when we can assume go1.18.
+func cut(s, sep string) (before, after string, found bool) {
+	if i := strings.Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
 	}
-	return strings.Contains(split[1], "/")
+	return s, "", false
 }
 
 // unappliedChanges is a file source that handles an uncloned snapshot.
@@ -1691,14 +1693,16 @@
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
-	// If there is an initialization error and a vendor directory changed, try to
-	// reinit.
-	if s.initializedErr != nil {
-		for uri := range changes {
-			if inVendor(uri) {
-				reinit = true
-				break
-			}
+	// Changes to vendor tree may require reinitialization,
+	// either because of an initialization error
+	// (e.g. "inconsistent vendoring detected"), or because
+	// one or more modules may have moved into or out of the
+	// vendor tree after 'go mod vendor' or 'rm -fr vendor/'.
+	for uri := range changes {
+		if inVendor(uri) && s.initializedErr != nil ||
+			strings.HasSuffix(string(uri), "/vendor/modules.txt") {
+			reinit = true
+			break
 		}
 	}
 
@@ -1782,7 +1786,7 @@
 	}
 
 	// directIDs keeps track of package IDs that have directly changed.
-	// It maps id->invalidateMetadata.
+	// Note: this is not a set, it's a map from id to invalidateMetadata.
 	directIDs := map[PackageID]bool{}
 
 	// Invalidate all package metadata if the workspace module has changed.
@@ -1821,7 +1825,7 @@
 		// Mark all of the package IDs containing the given file.
 		filePackageIDs := invalidatedPackageIDs(uri, s.meta.ids, pkgFileChanged)
 		for id := range filePackageIDs {
-			directIDs[id] = directIDs[id] || invalidateMetadata
+			directIDs[id] = directIDs[id] || invalidateMetadata // may insert 'false'
 		}
 
 		// Invalidate the previous modTidyHandle if any of the files have been
diff --git a/gopls/internal/lsp/cache/view.go b/gopls/internal/lsp/cache/view.go
index 54b4122..bcb6f3a 100644
--- a/gopls/internal/lsp/cache/view.go
+++ b/gopls/internal/lsp/cache/view.go
@@ -1128,28 +1128,34 @@
 // FileHandle from the cache for temporary files is problematic, since we
 // cannot delete it.
 func (s *snapshot) vendorEnabled(ctx context.Context, modURI span.URI, modContent []byte) (bool, error) {
+	// Legacy GOPATH workspace?
 	if s.workspaceMode()&moduleMode == 0 {
 		return false, nil
 	}
+
+	// Explicit -mod flag?
 	matches := modFlagRegexp.FindStringSubmatch(s.view.goEnv["GOFLAGS"])
-	var modFlag string
 	if len(matches) != 0 {
-		modFlag = matches[1]
-	}
-	if modFlag != "" {
-		// Don't override an explicit '-mod=vendor' argument.
-		// We do want to override '-mod=readonly': it would break various module code lenses,
-		// and on 1.16 we know -modfile is available, so we won't mess with go.mod anyway.
-		return modFlag == "vendor", nil
+		modFlag := matches[1]
+		if modFlag != "" {
+			// Don't override an explicit '-mod=vendor' argument.
+			// We do want to override '-mod=readonly': it would break various module code lenses,
+			// and on 1.16 we know -modfile is available, so we won't mess with go.mod anyway.
+			return modFlag == "vendor", nil
+		}
 	}
 
 	modFile, err := modfile.Parse(modURI.Filename(), modContent, nil)
 	if err != nil {
 		return false, err
 	}
+
+	// No vendor directory?
 	if fi, err := os.Stat(filepath.Join(s.view.rootURI.Filename(), "vendor")); err != nil || !fi.IsDir() {
 		return false, nil
 	}
+
+	// Vendoring enabled by default by go declaration in go.mod?
 	vendorEnabled := modFile.Go != nil && modFile.Go.Version != "" && semver.Compare("v"+modFile.Go.Version, "v1.14") >= 0
 	return vendorEnabled, nil
 }
diff --git a/gopls/internal/lsp/cache/view_test.go b/gopls/internal/lsp/cache/view_test.go
index 99daff1..f57fc80 100644
--- a/gopls/internal/lsp/cache/view_test.go
+++ b/gopls/internal/lsp/cache/view_test.go
@@ -111,18 +111,11 @@
 		path     string
 		inVendor bool
 	}{
-		{
-			path:     "foo/vendor/x.go",
-			inVendor: false,
-		},
-		{
-			path:     "foo/vendor/x/x.go",
-			inVendor: true,
-		},
-		{
-			path:     "foo/x.go",
-			inVendor: false,
-		},
+		{"foo/vendor/x.go", false},
+		{"foo/vendor/x/x.go", true},
+		{"foo/x.go", false},
+		{"foo/vendor/foo.txt", false},
+		{"foo/vendor/modules.txt", false},
 	} {
 		if got := inVendor(span.URIFromPath(tt.path)); got != tt.inVendor {
 			t.Errorf("expected %s inVendor %v, got %v", tt.path, tt.inVendor, got)
diff --git a/gopls/internal/regtest/misc/definition_test.go b/gopls/internal/regtest/misc/definition_test.go
index a5030d7..8f8619e 100644
--- a/gopls/internal/regtest/misc/definition_test.go
+++ b/gopls/internal/regtest/misc/definition_test.go
@@ -5,7 +5,9 @@
 package misc
 
 import (
+	"os"
 	"path"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -286,3 +288,87 @@
 		env.Editor.Server.Definition(env.Ctx, params)
 	})
 }
+
+// TestVendoringInvalidatesMetadata ensures that gopls uses the
+// correct metadata even after an external 'go mod vendor' command
+// causes packages to move; see issue #55995.
+func TestVendoringInvalidatesMetadata(t *testing.T) {
+	const proxy = `
+-- other.com/b@v1.0.0/go.mod --
+module other.com/b
+go 1.14
+
+-- other.com/b@v1.0.0/b.go --
+package b
+const K = 0
+`
+	const src = `
+-- go.mod --
+module example.com/a
+go 1.14
+require other.com/b v1.0.0
+
+-- go.sum --
+other.com/b v1.0.0 h1:1wb3PMGdet5ojzrKl+0iNksRLnOM9Jw+7amBNqmYwqk=
+other.com/b v1.0.0/go.mod h1:TgHQFucl04oGT+vrUm/liAzukYHNxCwKNkQZEyn3m9g=
+
+-- a.go --
+package a
+import "other.com/b"
+const _ = b.K
+
+`
+	WithOptions(
+		ProxyFiles(proxy),
+		Modes(Default), // fails in 'experimental' mode
+	).Run(t, src, func(t *testing.T, env *Env) {
+		// Enable to debug go.sum mismatch, which may appear as
+		// "module lookup disabled by GOPROXY=off", confusingly.
+		if false {
+			env.DumpGoSum(".")
+		}
+
+		env.OpenFile("a.go")
+		refPos := env.RegexpSearch("a.go", "K") // find "b.K" reference
+
+		// Initially, b.K is defined in the module cache.
+		gotFile, _ := env.GoToDefinition("a.go", refPos)
+		wantCache := filepath.ToSlash(env.Sandbox.GOPATH()) + "/pkg/mod/other.com/b@v1.0.0/b.go"
+		if gotFile != wantCache {
+			t.Errorf("GoToDefinition, before: got file %q, want %q", gotFile, wantCache)
+		}
+
+		// Run 'go mod vendor' outside the editor.
+		if err := env.Sandbox.RunGoCommand(env.Ctx, ".", "mod", []string{"vendor"}, true); err != nil {
+			t.Fatalf("go mod vendor: %v", err)
+		}
+
+		// Synchronize changes to watched files.
+		env.Await(env.DoneWithChangeWatchedFiles())
+
+		// Now, b.K is defined in the vendor tree.
+		gotFile, _ = env.GoToDefinition("a.go", refPos)
+		wantVendor := "vendor/other.com/b/b.go"
+		if gotFile != wantVendor {
+			t.Errorf("GoToDefinition, after go mod vendor: got file %q, want %q", gotFile, wantVendor)
+		}
+
+		// Delete the vendor tree.
+		if err := os.RemoveAll(env.Sandbox.Workdir.AbsPath("vendor")); err != nil {
+			t.Fatal(err)
+		}
+		// Notify the server of the deletion.
+		if err := env.Sandbox.Workdir.CheckForFileChanges(env.Ctx); err != nil {
+			t.Fatal(err)
+		}
+
+		// Synchronize again.
+		env.Await(env.DoneWithChangeWatchedFiles())
+
+		// b.K is once again defined in the module cache.
+		gotFile, _ = env.GoToDefinition("a.go", refPos)
+		if gotFile != wantCache {
+			t.Errorf("GoToDefinition, after rm -rf vendor: got file %q, want %q", gotFile, wantCache)
+		}
+	})
+}
diff --git a/internal/persistent/map.go b/internal/persistent/map.go
index f5dd102..b29cfe4 100644
--- a/internal/persistent/map.go
+++ b/internal/persistent/map.go
@@ -8,7 +8,9 @@
 package persistent
 
 import (
+	"fmt"
 	"math/rand"
+	"strings"
 	"sync/atomic"
 )
 
@@ -41,6 +43,18 @@
 	root *mapNode
 }
 
+func (m *Map) String() string {
+	var buf strings.Builder
+	buf.WriteByte('{')
+	var sep string
+	m.Range(func(k, v interface{}) {
+		fmt.Fprintf(&buf, "%s%v: %v", sep, k, v)
+		sep = ", "
+	})
+	buf.WriteByte('}')
+	return buf.String()
+}
+
 type mapNode struct {
 	key         interface{}
 	value       *refValue