As far as I know, Adobe Illustrator files support both raster-based and vector-based images; it's possible you already *have* a vector. In the "Save as" and/or "Export" menu, see if you can select SVG (or "Scalable Vector Graphics" or anything mentioning "vector" really). If so, do that and ignore the rest of this post.
Converting a vector image to a raster image (SVG to JPG/PNG/BMP/...) is easy. The other way around is a lot harder, *but it can be done* (well, sort of). Especially if the raster is high quality and preferably only has solid colours (no beautiful-looking-but-software-confusing gradients and stuff like that). Some links that might help:
- [Vector Magic](http://vectormagic.com/home
) - a website which lets you upload a raster image, play with some settings and creates an SVG automatically.
) - a program to do pretty much the same thing on your own computer (available for both Windows and Linux).
- [Adobe CS2](http://www.adobe.com/designcenter-archive/illustrator/articles/illcs2ttlvtrace/illcs2ttlvtrace.pdf
) - an explanation on how to do this in Adobe itself. I guess they wouldn't remove this feature once it's there, so this probably should still work in CS5 (or whatever you have).
) has a comparison of different programs capable of this trick.
- You can always Google (suggested search terms "bitmap to vector" or "raster to vector") for more information.