I guess I am old school LOL - whenever I need to mask something, I take copy of the frame in question, paste it into Gimp (or Photoshop) and then create the mask by selecting the area I want to show through the mask, fill that with white, then create a black layer under that and save it as a PNG (remove the original frame copy from Vegas as that was just a guide).
I then use the image and create a mask track over the video. Gets me the exact mask I want quickly.