If it's always right around 4% regardless of the material, then the first thing I would do is check for dust on the CPU heat sink and CPU fan (and make sure the fan is working). Rendering is very processor intensive, which creates more heat - if your processor is running on the edge heat-wise, rendering will push it off the cliff.
Use some canned air to blow off any dust, also dust the motherboard and the power supply.
FWIW there are some posts online re: crashes rendering etc. -- often the cause is very elusive. If it helps narrow things down at all, you can eliminate whatever apps (in this case Vegas) by downloading & running Prime95: http://en.wikipedia.org/wiki/Prime95
Simply, if it crashes -> don't look specifically at Vegas, but at your system in general. If it doesn't, look at Vegas or the disc you're rendering to and so on.
Heat is a problem with almost every or even *any* current processor. Accumulated dust certainly can make it worse as Scott pointed out, but an awful lot has to do with case & heatsink design -- number of fans & placement is important. Current CPUs will often more or less idle until some task causes them to *floor it*, & that's when the CPU & M/Board temps rise. You can monitor it using utilities like MBM (Motherboard Monitor), or with hardware displays. There's plenty of info online, especially at any site concerning overclocking.