diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index d1cd520245..c74ba5f63c 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -961,6 +961,8 @@ func disallowInternal(srcDir string, p *Package, stk *ImportStack) *Package {
 	if i > 0 {
 		i-- // rewind over slash in ".../internal"
 	}
+
+	var where string
 	if p.Module == nil {
 		parent := p.Dir[:i+len(p.Dir)-len(p.ImportPath)]
 
@@ -978,19 +980,18 @@ func disallowInternal(srcDir string, p *Package, stk *ImportStack) *Package {
 		// p is in a module, so make it available based on the import path instead
 		// of the file path (https://golang.org/issue/23970).
 		parent := p.ImportPath[:i]
-		// TODO(bcmills): In case of replacements, use the module path declared by
-		// the replacement module, not the path seen by the user.
-		importerPath := (*stk)[len(*stk)-2]
-		if strings.HasPrefix(importerPath, parent) {
+		importer := (*stk)[len(*stk)-2]
+		if str.HasPathPrefix(importer, parent) {
 			return p
 		}
+		where = " in " + importer
 	}
 
 	// Internal is present, and srcDir is outside parent's tree. Not allowed.
 	perr := *p
 	perr.Error = &PackageError{
 		ImportStack: stk.Copy(),
-		Err:         "use of internal package " + p.ImportPath + " not allowed",
+		Err:         "use of internal package " + p.ImportPath + " not allowed" + where,
 	}
 	perr.Incomplete = true
 	return &perr
@@ -1027,6 +1028,29 @@ func disallowVendor(srcDir, path string, p *Package, stk *ImportStack) *Package
 		return p
 	}
 
+	if p.Standard && ModPackageModuleInfo != nil {
+		// Modules must not import vendor packages in the standard library,
+		// but the usual vendor visibility check will not catch them
+		// because the module loader presents them with an ImportPath starting
+		// with "golang_org/" instead of "vendor/".
+		importer := (*stk)[len(*stk)-2]
+		if mod := ModPackageModuleInfo(importer); mod != nil {
+			dir := p.Dir
+			if relDir, err := filepath.Rel(p.Root, p.Dir); err == nil {
+				dir = relDir
+			}
+			if _, ok := FindVendor(filepath.ToSlash(dir)); ok {
+				perr := *p
+				perr.Error = &PackageError{
+					ImportStack: stk.Copy(),
+					Err:         "use of vendored package " + path + " not allowed",
+				}
+				perr.Incomplete = true
+				return &perr
+			}
+		}
+	}
+
 	if perr := disallowVendorVisibility(srcDir, p, stk); perr != p {
 		return perr
 	}
diff --git a/src/cmd/go/testdata/script/mod_internal.txt b/src/cmd/go/testdata/script/mod_internal.txt
index 5ad392c088..bbc84b168c 100644
--- a/src/cmd/go/testdata/script/mod_internal.txt
+++ b/src/cmd/go/testdata/script/mod_internal.txt
@@ -1,16 +1,21 @@
 env GO111MODULE=on
 
 # golang.org/x/internal should be importable from other golang.org/x modules.
+rm go.mod
 go mod -init -module golang.org/x/anything
 go build .
 
 # ...but that should not leak into other modules.
 ! go build ./baddep
-stderr 'use of internal package'
+stderr 'use of internal package golang.org/x/.* not allowed in golang.org/notx/useinternal$'
 
 # Internal packages in the standard library should not leak into modules.
 ! go build ./fromstd
-stderr 'use of internal package'
+stderr 'use of internal package internal/testenv not allowed$'
+
+# Packages found via standard-library vendoring should not leak.
+! go build ./fromstdvendor
+stderr 'use of vendored package golang_org/x/net/http/httpguts not allowed$'
 
 
 # Dependencies should be able to use their own internal modules...
@@ -20,11 +25,27 @@ go build ./throughdep
 
 # ... but other modules should not, even if they have transitive dependencies.
 ! go build .
-stderr 'use of internal package'
+stderr 'use of internal package golang.org/x/.* not allowed in golang.org/notx$'
 
 # And transitive dependencies still should not leak.
 ! go build ./baddep
-stderr 'use of internal package'
+stderr 'use of internal package golang.org/x/.* not allowed in golang.org/notx/useinternal$'
+
+
+# Replacing an internal module should keep it internal to the same paths.
+rm go.mod
+go mod -init -module golang.org/notx
+go mod -replace golang.org/x/internal=./replace/golang.org/notx/internal
+go build ./throughdep
+
+! go build ./baddep
+stderr 'use of internal package golang.org/x/.* not allowed in golang.org/notx/useinternal$'
+
+go mod -replace golang.org/x/internal=./vendor/golang.org/x/internal
+go build ./throughdep
+
+! go build ./baddep
+stderr 'use of internal package golang.org/x/.* not allowed in golang.org/notx/useinternal$'
 
 
 -- useinternal.go --
@@ -42,3 +63,21 @@ import _ "golang.org/notx/useinternal"
 -- fromstd/useinternal.go --
 package fromstd
 import _ "internal/testenv"
+
+-- fromstdvendor/useinternal.go --
+package fromstdvendor
+import _ "golang_org/x/net/http/httpguts"
+
+-- replace/golang.org/notx/internal/go.mod --
+module golang.org/x/internal
+
+-- replace/golang.org/notx/internal/subtle/subtle.go --
+package subtle
+// Ha ha! Nothing here!
+
+-- vendor/golang.org/x/internal/go.mod --
+module golang.org/x/internal
+
+-- vendor/golang.org/x/internal/subtle/subtle.go --
+package subtle
+// Ha ha! Nothing here!