VP10 was definitely available in both 32 and 64 bits - it's in the VP10 user manual. I recall that VP11 was also available in both 32 and 64 bits - and I just found a youtube video tuturiol on line that shows the SCS download page for VP11 at the time of its release - and both 32 and 64 bit version were available at VP11's release date. As I recall, VP12 was 64 bits only.
VP11 build 700 was the last 32 bit (701 was 64 bits). The following page still loads from SCS's website and VP11 32 and 64 is there, but I don't know if the downloads will work now - give it a try anyway:
the next question would be what is the last most stable 32bit version, thanks :P
Vegas Pro 8.0c was rock solid. I loved that version and got so much done with it. But it won't support formats such as long-GOP AVC as well as later versions will. Although some say that the mature versions of V9 were stable, there was at least one report that the dreaded replaced-media bug in V10 also appeared earlier in V9. By V11 that horror bug had mostly gone (but apparently not completely), but then the new GPU-acceleration played havoc with stability in that version.
But it also depends on the OS. I never saw MSP12 for winXP-32. It may be that MSP12 was available as 32 bit but only for win7 and up. That may be relevant to the OP. I have several winXP-32 machines and have no appetite for reinstalling 100+ apps :)
Also, there is no lens correction plug-in for any Vegas version which is 32-bit. That is why I finally went to win7-64 (and Pro 13) for my main PC. This may well be relevant if someone is using action cam footage because they all distort horribly. All the 32-bit plug-ins (e.g. DEFISHR) were standalone apps.
I found MSP11 to be utterly stable, for the several years I used it. MSP12 also (win7-64). Pro 13, much less so, but then I am doing more with it.