I am making an extension for both VS2010 and VS2012 that has a toolbar command.
Given the change in icon style between them, I want to provide a separate icon for each.
How do I specify that in .vsct? I assume I might be able to use Condition, but I am not sure if there is already a pre-defined value with VS version.
It seems it is not possible without using an MSI, see "VS Package (VSIX): Having two different toolbar icons for VS2010/VS2012" on MSDN forums.
I do not believe you can use VSIX to install or specify target specific resources. VSIX is intended as a very simplistic deployment mechanism, it is in fact just a renamed ZIP file plus an installer with a small amount of logic to process pkgdef files. If you want more complex installs (like laying down different dlls based on your install target) I believe you would need to create an MSI.