I am not sure if anyone has made any progress here, I had the same issue on windows 2012; I downloaded a copy of the source code for the vb6 task from the 4.0.10 release.
I temporarily removed the redirection of stdout and sterr, the vb6 compiler does not throw anything much anyways as long as a log file is provided (which the plugin specified by default).
Extract looks like this
proc.StartInfo.FileName = this.VB6Path;
proc.StartInfo.UseShellExecute = false;
proc.StartInfo.RedirectStandardOutput = false;
proc.StartInfo.RedirectStandardError = false;
I also removed related code that would read the streams to the end and post as Log.Message or Log.Error; after that I reran my build and now VB6 does not hang.
I guess this will be my workaround while this is officially supported on Windows 2012.