I hope so. But the cynic in me thinks think there won't be as much work because now most people who would work on homebrew games and emulators are now coding for smartphones and tablets. However, by getting an open sdk started and releasing a public exploit for developers (honestly it's not user friendly enough for everyone), I'm hoping the scene will finally get a jump start.

I only bought a DS when there was homebrew available - or at least the possibility of "Hello World". Is the Vita at that point now? Do I have to search eBay for an old Vita or will the newest ones work?

Don't bother. But to answer somewhat your question: You should look for one with a game on it, which is known to be usable for hacking. Those games are mostly pulled from the store and not available anymore.

In a sense we already literally do: the only working exploits (that I know of, it's been a while since I checked) involve the PSP emulator on the system. So you have the same activity insofar as you can run the exact same homebrew :P