I've used http://www.geisswerks.com/drempels/ which is a screensaver to good effect. To put it on the Vegas timeline, I used the TV-out desktop clone function on my gfx card. (then bring it back in via DV) Possibly WMEnc and other desktop capture tools (techsmith etc) can also capture the effort.
Slightly less plasma than I think you might be wanting? But free, if painful to realise/render.
Just experiment with generated noise textures right inside Vegas. No need for another program to create plasmas.
Right click on video track -> insert generated media -> SF noise texture. Choose a preset texture, add a keyframe at the end of the clip with new "progress" value, and enjoy.
You might want to check out particleIllusion SE 2.0. You can download a 30-day trial from WonderTouch.com. It’s a particle emitter that can do a whole lot more than just plasma backgrounds but it does those great as well. If you ever needed to add smoke, fire, water, space explosions and more to your videos, you’ll find it a great tool. If you get the non-demo version there are lots of sample libraries on their web site and you’ll find lots of swirls, tunnels, lava, and other plasma effects in those libraries.
The great part is you can get it for around $15 USD if you order a back issue of Digit Magazine #64. It comes on the CD with the magazine (and the magazine is pretty good too). That’s how I got a copy. You might want to watch the web sites of both Digit and 3D World magazine as they both have some great full-featured programs for free on their monthly CD’s. I also got Cinema 4D CE6 this way.