Hmmm... I think I've seen something like this in the past. It's kind of a mistery, and it seems it can be solved tweaking things around. Someone got it to work by disabling/reenabling Visual Assist:
http://www.viemu.com/forums/viewtopic.php?id=89
Can you verify whether that helps? Maybe uninstalling Visual Assist, starting VS to force cleanup, and installing it back could help too. Reinstalling ViEmu may also help (the order of 3rd party add-in installation seems to affect things, even if it shouldn't).
Input dispatch in Visual Studio is quite messy, there is no "official" key hooking mechanism and all 3rd party add-ins have to resort to window subclassing, which can result in some weird results at times.
Let me know if any of that helps.