If the camera is still new, or to say at least under warranty then definitely get it looked at/sent in by professionals. If you're comfortable with taking it apart then understand that you're dealing with optical paths and will be tricky putting it all back together (if you can take it apart properly that is), the 600D has a Penta-mirror for the VF, pretty sure it'll mostly be mirrors/glass that have been pasted together, but you may not need to go that far.
The issue sounds like it's with the diopter, with a finger pushed in there, it must have come loose and got pushed too far in? That's why adjusting doesn't do anything, as it's not moving at all possibly.