UNC paths should have backslashes, not front-slashes.

lib/vagrant/util/platform.rb

@@ -148,7 +148,7 @@ module Vagrant
         # @param [String] path Path to convert to UNC for Windows
         # @return [String]
         def windows_unc_path(path)
-          "//?/" + path.gsub("/", "\\")
+          "\\\\?\\" + path.gsub("/", "\\")
         end
 
         # Returns a boolean noting whether the terminal supports color.

test/unit/vagrant/util/platform_test.rb

@@ -13,7 +13,7 @@ describe Vagrant::Util::Platform do
 
   describe "#windows_unc_path" do
     it "correctly converts a path" do
-      expect(described_class.windows_unc_path("c:/foo").to_s).to eql("//?/c:\\foo")
+      expect(described_class.windows_unc_path("c:/foo").to_s).to eql("\\\\?\\c:\\foo")
     end
   end
 end