The condition for setting property VSToolsPath in MSBuild.Microsoft.VisualStudio.Web.targets.props does not successfully compare with an empty string when the VSToolsPath property is empty. $(VSToolsPath) should be wrapped in single quotes as shown below:
<VSToolsPath Condition="'$(VSToolsPath)' == ''" >$(MSBuildThisFileDirectory)\..\tools\VSToolsPath</VSToolsPath>