diff --git a/src/cmd/go/internal/modload/build.go b/src/cmd/go/internal/modload/build.go
index a506c25dc7eb7c..b87a7002567a76 100644
--- a/src/cmd/go/internal/modload/build.go
+++ b/src/cmd/go/internal/modload/build.go
@@ -145,34 +145,38 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 				}
 			}
 		}
-		if cfg.BuildMod == "vendor" {
-			m.Dir = filepath.Join(ModRoot, "vendor", m.Path)
-		}
 	}
 
-	complete(info)
+	if !fromBuildList {
+		complete(info)
+		return info
+	}
 
-	if fromBuildList {
-		if r := Replacement(m); r.Path != "" {
-			info.Replace = &modinfo.ModulePublic{
-				Path:      r.Path,
-				Version:   r.Version,
-				GoVersion: info.GoVersion,
-			}
-			if r.Version == "" {
-				if filepath.IsAbs(r.Path) {
-					info.Replace.Dir = r.Path
-				} else {
-					info.Replace.Dir = filepath.Join(ModRoot, r.Path)
-				}
-			}
-			complete(info.Replace)
-			info.Dir = info.Replace.Dir
-			info.GoMod = filepath.Join(info.Dir, "go.mod")
-			info.Error = nil // ignore error loading original module version (it has been replaced)
-		}
+	r := Replacement(m)
+	if r.Path == "" {
+		complete(info)
+		return info
 	}
 
+	// Don't hit the network to fill in extra data for replaced modules.
+	// The original resolved Version and Time don't matter enough to be
+	// worth the cost, and we're going to overwrite the GoMod and Dir from the
+	// replacement anyway. See https://golang.org/issue/27859.
+	info.Replace = &modinfo.ModulePublic{
+		Path:      r.Path,
+		Version:   r.Version,
+		GoVersion: info.GoVersion,
+	}
+	if r.Version == "" {
+		if filepath.IsAbs(r.Path) {
+			info.Replace.Dir = r.Path
+		} else {
+			info.Replace.Dir = filepath.Join(ModRoot, r.Path)
+		}
+	}
+	complete(info.Replace)
+	info.Dir = info.Replace.Dir
+	info.GoMod = filepath.Join(info.Dir, "go.mod")
 	return info
 }
 
diff --git a/src/cmd/go/testdata/script/mod_replace.txt b/src/cmd/go/testdata/script/mod_replace.txt
index 5894ed69f34178..b9cf00c36cb5b5 100644
--- a/src/cmd/go/testdata/script/mod_replace.txt
+++ b/src/cmd/go/testdata/script/mod_replace.txt
@@ -1,10 +1,14 @@
 env GO111MODULE=on
 
+cp go.mod go.mod.orig
+
+# Make sure the test builds without replacement.
 go build -o a1.exe .
 exec ./a1.exe
 stdout 'Don''t communicate by sharing memory'
 
 # Modules can be replaced by local packages.
+cp go.mod.orig go.mod
 go mod edit -replace=rsc.io/quote/v3=./local/rsc.io/quote/v3
 go build -o a2.exe .
 exec ./a2.exe
@@ -12,16 +16,26 @@ stdout 'Concurrency is not parallelism.'
 
 # The module path of the replacement doesn't need to match.
 # (For example, it could be a long-running fork with its own import path.)
+cp go.mod.orig go.mod
 go mod edit -replace=rsc.io/quote/v3=./local/not-rsc.io/quote/v3
 go build -o a3.exe .
 exec ./a3.exe
 stdout 'Clear is better than clever.'
 
 # However, the same module can't be used as two different paths.
-go mod edit -dropreplace=rsc.io/quote/v3 -replace=not-rsc.io/quote/v3@v3.0.0=rsc.io/quote/v3@v3.0.0 -require=not-rsc.io/quote/v3@v3.0.0
+cp go.mod.orig go.mod
+go mod edit -replace=not-rsc.io/quote/v3@v3.0.0=rsc.io/quote/v3@v3.0.0 -require=not-rsc.io/quote/v3@v3.0.0
 ! go build -o a4.exe .
 stderr 'rsc.io/quote/v3@v3.0.0 used for two different module paths \(not-rsc.io/quote/v3 and rsc.io/quote/v3\)'
 
+# Modules that do not (yet) exist upstream can be replaced too.
+cp go.mod.orig go.mod
+go mod edit -require not-rsc.io/quote/v3@v3.0.0 -replace=not-rsc.io/quote/v3=./local/rsc.io/quote/v3
+go build -o a5.exe ./usenewmodule
+! stderr 'finding not-rsc.io/quote/v3'
+exec ./a5.exe
+stdout 'Concurrency is not parallelism.'
+
 -- go.mod --
 module quoter
 
@@ -39,6 +53,18 @@ func main() {
 	fmt.Println(quote.GoV3())
 }
 
+-- usenewmodule/main.go --
+package main
+
+import (
+	"fmt"
+	"not-rsc.io/quote/v3"
+)
+
+func main() {
+	fmt.Println(quote.GoV3())
+}
+
 -- local/rsc.io/quote/v3/go.mod --
 module rsc.io/quote/v3
 
diff --git a/src/cmd/go/testdata/script/mod_vendor_replace.txt b/src/cmd/go/testdata/script/mod_vendor_replace.txt
new file mode 100644
index 00000000000000..6bc1c77ed3d0d7
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_replace.txt
@@ -0,0 +1,39 @@
+env GO111MODULE=on
+
+# Before vendoring, we expect to see the original directory.
+go list -f '{{.Version}} {{.Dir}}' -m rsc.io/quote/v3
+stdout 'v3.0.0'
+stdout '.*[/\\]not-rsc.io[/\\]quote[/\\]v3'
+
+# Since all dependencies are replaced, 'go mod vendor' should not
+# have to download anything from the network.
+go mod vendor
+! stderr 'downloading'
+! stderr 'finding'
+
+# After vendoring, we expect to see the replacement in the vendor directory,
+# without attempting to look up the non-replaced version.
+cmp vendor/rsc.io/quote/v3/quote.go local/not-rsc.io/quote/v3/quote.go
+
+go list -mod=vendor -f '{{.Version}} {{.Dir}}' -m rsc.io/quote/v3
+stdout 'v3.0.0'
+stdout '.*[/\\]vendor[/\\]rsc.io[/\\]quote[/\\]v3'
+! stderr 'finding'
+! stderr 'lookup disabled'
+
+-- go.mod --
+module example.com/replace
+
+require rsc.io/quote/v3 v3.0.0
+replace rsc.io/quote/v3 => ./local/not-rsc.io/quote/v3
+
+-- imports.go --
+package replace
+
+import _ "rsc.io/quote/v3"
+
+-- local/not-rsc.io/quote/v3/go.mod --
+module not-rsc.io/quote/v3
+
+-- local/not-rsc.io/quote/v3/quote.go --
+package quote