Interesting. I have to admit I have no idea what version of BitDefender is on my system. That's not quite true, but ....
According to the "Uninstall or Change a Program" display from the Control Panel, I've been running "BitDefender Antivirus Free Edition" since 3/11/2017. It;s a 293 MB program with version number 1,0.21.1109. Which is the same version number I see when I click on the BitDefender icon on my toolbar. When I bring up, as I just did, Bitdefender assures me I am logged in to their system, that the system is working, and that it updated today, about an hour ago.
That said .. when I go to Major Geeks and look for Bitdefender Antivirus, the newest thing I find is Bitdefender Online version 18.104.22.168. Which is a 10 MB file -- an installer, obviously -- dating back to 6/29/2017. Not exactly that new! Going by dates on the files on my system, I suspect I installed from a thing called Bitdefender Online version 22.214.171.124, an 8MB file dated to 12/2/2016.
And at the BitDefender website, I find a somewhat newer (02/06/2018) and somewhat larger (still about 10 MB) and unnumbered installer for a free versionm also called Bitdefender_Online, which ... You're right ... won't install anything on a Vista system. However click on it anyway, and you'll notice the refusal note suggests Vista and XP users can click on an OK button to look for more options. Do that, and you'll go to a website suggesting you obtain a Bitdefender subscription -- buy something, in other words -- and then change your suscription info from say a Win 10 user to a Vista user. Likely that's not your idea of fun, but at the bottom of the page, you'll see you have yet another option, which is to download another installer, a 3MB file named bitdefender_isecurity.exe from 02/25/2015.
You're following all this? I suggest you try that bitdefender_isecurity.exe file. I can't go any further experimenting myself because the next thing it wants to do on my system is uninstall my current Bitdefender setup, and I think that leads to more work than I want to do. But I think it gives you a shot at a decent for free version of Bitdefender, and bringing it up to date is straightforward.