I'm pretty sure Sonar 6 was the first to support 64 bit, so yes, it will have been recorded in 32 bit, and not support bitbridge.
The OP has pointed out that the offending plug in was only added in X2, and was not present in the original project.
I do think that it is picking up the 32 bit version of Vocal Strip. I only use X2x64, so I'm not sure how it works with both installed on your system.
Have a look in your plug in manager to see if Sonar is also scanning your 32 bit version of this plug.
I have loaded projects from Sonar 6 32 bit and not experienced this sort of issue.