diff --git build/toolchain/win/setup_toolchain.py build/toolchain/win/setup_toolchain.py
index 927fce24fcdc6..834c84eae805e 100644
--- build/toolchain/win/setup_toolchain.py
+++ build/toolchain/win/setup_toolchain.py
@@ -166,13 +166,17 @@ def _LoadToolchainEnv(cpu, toolchain_root, sdk_dir, target_store):
           del os.environ['LIB']
         if 'LIBPATH' in os.environ:
           del os.environ['LIBPATH']
-      other_path = os.path.normpath(os.path.join(
+      script_path = os.path.normpath(os.path.join(
                                         os.environ['GYP_MSVS_OVERRIDE_PATH'],
                                         'VC/Auxiliary/Build/vcvarsall.bat'))
-      if not os.path.exists(other_path):
-        raise Exception('%s is missing - make sure VC++ tools are installed.' %
-                        script_path)
-      script_path = other_path
+    if not os.path.exists(script_path):
+      # Compiler environment variables must already be specified.
+      variables = []
+      for k in sorted(os.environ.keys()):
+        variables.append('%s=%s' % (str(k), str(os.environ[k])))
+      variables = '\n'.join(variables)
+      return _ExtractImportantEnvironment(variables)
+
     cpu_arg = "amd64"
     if (cpu != 'x64'):
       # x64 is default target CPU thus any other CPU requires a target set
diff --git build/vs_toolchain.py build/vs_toolchain.py
index f0b8bacf57360..585e64876f44e 100755
--- build/vs_toolchain.py
+++ build/vs_toolchain.py
@@ -136,9 +136,16 @@ def SetEnvironmentAndGetRuntimeDllDirs():
     runtime_path = os.path.pathsep.join(vs_runtime_dll_dirs)
     os.environ['PATH'] = runtime_path + os.path.pathsep + os.environ['PATH']
   elif sys.platform == 'win32' and not depot_tools_win_toolchain:
+    has_override_path = True
     if not 'GYP_MSVS_OVERRIDE_PATH' in os.environ:
+      has_override_path = False
       os.environ['GYP_MSVS_OVERRIDE_PATH'] = DetectVisualStudioPath()
 
+    if has_override_path:
+      # Don't attempt to copy DLLs when using a custom toolchain.
+      # The DLLs should already be discoverable via the PATH env variable.
+      return None
+
     # When using an installed toolchain these files aren't needed in the output
     # directory in order to run binaries locally, but they are needed in order
     # to create isolates or the mini_installer. Copying them to the output
@@ -187,6 +194,10 @@ def _RegistryGetValue(key, value):
 def GetVisualStudioVersion():
   """Return best available version of Visual Studio.
   """
+  # Return the explicitly requested version, if any.
+  if 'GYP_MSVS_VERSION' in os.environ:
+    return os.environ['GYP_MSVS_VERSION']
+
   supported_versions = list(MSVS_VERSIONS.keys())
 
   # VS installed in depot_tools for Googlers