There are a few things that can cause this, but you've left out important info that could help pinpoint the problem quicker. It's usually a good idea to describe your computer system when posing a computer question (ie, processor, processor speed, amount of RAM, operating system, etc.). Here's what I'd look at, though, and forgive me if any are obvious.
1. Not enough RAM. How much do you have? The more the better with Vegas. You can set Vegas to read ahead up to 2 seconds and that will help some, but that will also create lag time between making a change and hearing the change in real time.
2. Slow processor. My old computer ran at 400 MHz, and it used to start skipping and stuttering at around 20 tracks. My new computer runs at 2 GHz (800 MHz FSB). No more skipping even with 30+ tracks.
3. Too many FX on. FX eat up processing time, and the slower the processor, the more likely it will skip.
4. Badly fragmented hard drive. Do you defrag? It's a must. Also how big is the drive and how full is it? Some drives seem to operate better when they're not near capacity. Also, it's a good idea to have a second hard drive that is dedicated to audio files. Most agree that you don't want your audio on your main hard drive.
5. Running anti-virus software while using Vegas. Also there are other programs that run in the background that can interfere with audio.
And it's likely that it's not just one of these things, but a combination of two or more.