I have to install Sonar on my old (2007) Core 2 Duo system to use for rehearsals, and it runs Windows Vista 32 bit. I have Sonar Producer 8 and all the way up to Platinum, so any version is available. Which one would be the best? Thank you.

I would install Platinum.

I think Vista support ended with X2. I was certainly using X2 on my old Vista 64 DAW. At some point Sonar started to rely on Microsoft libraries that are not available in Vista.

I think the original install of X2 was ok - Vista support stopped after the first update to X2

To the best of my awareness, you cannot install anything past X2A on Vista, due to code dependencies in Sonar that require Windows maintenance that did not come into existence until Windows 7 Service Pack 1, so that is the minimum Windows required for anything past X2A.
 
Bob Bone
 

None the less I recall posts from many saying they had installed X3 on Vista. I don't think trying to install Platinum would hurt and one would know for sure.   

I DID find the following post from scook, indicating X3 would run on Vista:
 
http://forum.cakewalk.com/FindPost/2942295
 
But, at the same time, my earlier post seems quite vivid in my mind, and 2 others above seem to concur, so I am not sure.
 
Bob Bone
 

Nor am I Bob but it can't hurt to try.

Understood - I am still trying to see if I can find any info on it that is definitive, but it is quite the pain in the rump trying to ferret it out.
 
I did come across the following review of X3, which indicates Windows 7 or higher is required:
http://www.musictech.net/2013/10/sonar-x3-review/
 
But I also have a high degree of respect for scook, and he clearly indicated that it can run on Vista.  I guess I will leave it at the 'mebbe yup, and mebbe nope' and wish them luck with whatever version they are able to get up and running on Vista.
 
Bob Bone
 

One big reason CW doesn't support Vista is Sonar's touch implementation. If that is not a consideration Vista should work.