You might be able to work something out using noise textures, gradients, cookie cutters, lots of layers, etc...
If you are willing to take a look at a separate program that can generate those kinds of effects as image sequences, that you can then import into Vegas, take a look at particleIllusion (http://www.wondertouch.com/). There is now an SE version that is available for a reasonable price.
Or check out AlamDV. Again, it is something to do outside of VV and then import the final product.
Or....if you have the footage, you can layer a film clip of fire - cookie cut a ball - track motion it's movement and/or growth. Very easy to do (if you have the clip).
Particle Illusion is really sweet for making explosions I made a music video where a guy exploded and then I shot him in front of a green screen and created a fire storm with a vortex. Very cool for all kinds of particle effects like water smoke, fire, lightning sparks & lots of other cool stuff too. It allows you to import avi's and you can sync them there if you like. I bought Version 3 beta for like 270 dollars since it also has super emitters. But they have a light version for like 99 dollars. I like it better than Alam3d it allows you to create your own effects more than Alam does. It is also a 2d program but don't let that scare you away it is much faster and you can simulate 3d movements very easily. I rendered a 1 minute video in about 1 1/2 minutes full frames uncompresed and just a little longer for compresed video. I showed the people I did the music video for and they were blown away.
Check out there site way cool stuff .
If you've got more time then money, Quicktime itself has a few media generators and one of them does flames... Set your param., create the rough footage, and follow earlier suggestions.