If you're using Screenblast to make commercial stuff, I mean - to make money from in a business, it probably has some things in the end user license agreement you want to be aware of before a purchase. Do a search in this forum, I think there's several threads about it.
It's not unreasonable for a compositing tool to provide some flash functionality. After Effects does this. But After Effects isn't an NLE.
Still, Vegas could output track motion to a flash file. Or to an MPEG4 file, I suppose.
It's not a primary function of Vegas, though. Here I'd probably be in the BillyBoy camp-if it doesn't directly relate to Vegas and editing I'd leave it for third parties. It really depends on how much use it would be. If it brought more people into Vegas that's great.