Once you’ve finished editing, you can just export your PAL project to NTSC, doing the conversion directly from Vegas. However, I’ve found earlier versions of Vegas don’t do this neither fast nor with good quality. I don’t know about current versions. Since you’re using PC, I’d suggest rendering your project in PAL and then converting it using Canopus Procoder or Adobe Media Encoder. If you already have that software, you might be able to do a software conversion on your own. You’ll need to check the quality, though, as, from a purist point of view, it might not be good enough. Test it, though.