For your 64-bit computer, you could use the com0com null-modem emulator - a 64-bit signed version is available on the SourceForge.
Since I have had the Vantage Pro2 weather station, I have used Virtual VP. As far as I can remember, I have always had problems whenever I ran the WeatherLink software - it has not been designed to ‘play’ nice with any other software, including Virtual VP - it appears that the WeatherLink software does not like to have its input redirected through a software, it prefers direct connection to the data logger.