I'm assuming that you want to convert the PAL files to NTSC after you capture them. If so, it's very easy. Just use an NTSC template in Movie Studio (or TMPEGenc, or CCE, or another rendering program) to output the NTSC file from the PAL AVI file. It's slow, but it works with no problems.
If your converter hardware supports PAL then you're all set. Your computer and Vegas Movie Studio don't really care which format the video is. Once it's captured you can work with either format with equal ease. Capture and edit in PAL, then render the output as NTSC.