I'm not an expert on this, but it appears the 3gp format is for cell phones. Most cell phones have a version of it.
I don't believe Vegas has the necessary codecs to handle the rendering or compression, but you can find a couple of great utilities to convert almost any video or audio format. The "inside" of these utilities use are public domain, but the people have found nice "packages" to hold everything and work as a menu. Both have various bugs, but with some trial and error, you can usually convert what you want.
Below are the links. Mediacoder has a lot of options, while Super is simpler to use, but restricted in options. Both work well and fast in converting various formats, but there are some issues with audio/video sync. Both have very limited instructions or FAQ's, but they have forum support. Good luck.
Vegas 7 can't see my .3gp, recordered by Siemens S75 mobil phone camera.
Vegas 6 can see it as 125.000 fps, but it really 12.5 fps
QuickTime 7.0.3 Pro installed