I've long struggled with getting this configured and reconfigured following system crashes or installations on new computers and I was never able to identify one exact step to make it work and work effectively. At one point I could import audio and video but my clips couldn't exceed 45 seconds in length.
I set out to find a solution and tried an number of ones that did not work and stepped back and uninstalled any programs or codecs once failure was determined. The following solution worked on my Windows 10 64 Bit computer running Vegas Movie Studio Platinum 13. It is such and easy and lean solution that it makes me wonder why it has been so difficult to identify. There are a lot of just plain wrong or stupid suggestions out there on youtube and the web. Apparently, you can also cause some functionality issues with Vegas if you install codec packs that overwrite the default codecs for Vegas. When I say this solution is lean, it is because you are installing the Huffyuv codec and only the Huffyuv codec. It's a much more effective solution than rendering in Webm or even Huffuy from Moviestorm and converting the video in another application. I successfully imported a 3 minute video into Vegas without any audio or video synch issues.
The title says Windows 7 but it worked for me on Windows 10. I'd love to hear from anyone else that's struggled with this as to whether this solution works for them.