Really depends on what you have in mind. Like a fireball when those planes hit the WTC? Something less dramatic or something along the lines of a H bomb?
To be truly effective it probably needs animation, something that without some 3rd party application becomes difficult if not impossible in Vegas alone. If you've been following the forum for a few weeks or so you should have seen references to the free copy of Particlelllusion that came with Digit Magazine which does exactly that kind of thing.
Just got my back issue order from DIGIT magazine outa London. Loaded up the free ParticleIllusion SE and wow! Working through the tutorials right now. It will definitely do your fireball, and much much easier than trying to create something from scratch in animation or Vegas. DGrob