Believe it or not, MSE can still be downloaded and installed on Vista. I've attached screenshots. Oddly enough, I haven't gotten a nag screen, even after definition updates.
Screenshot notes:
Microsoft's website still lists Vista for the download, not just 7.
The installer did, indeed work.
Just to verify that it is Vista, I've inserted a screenshot of my system specs.
And, lastly, the application running on Vista after a reboot. Not to mention, I tried this out with an older installer on an XP machine the other day and it still could fetch the newest definitions.