In principal there is no reason why you can's use 32 bit VST plugins in 64 bit Sonar, however as scook says some do not work well, Bitbridge is automatically engaged when you try to load a 32 bit VST, unless you have JBridge and have told Sonar's plug in manager to use JBridge for that plugin.
In my experience the ones that dont work well with Bitbridge are those that load samples or poorly written cheap/free stuff.
This was a pretty hot topic back in the days when 64 bit applications were less well developed. Here is a thread I started in 2009 which still may contain some useful information
http://forum.cakewalk.com/FindPost/1829581 32 bit DXi plugins are another matter, they will not run in 64 bit Sonar unless you use some sort of DXi- VST wrapper.