Vacuums don't work very well for this. Part of the problem is that the hoses are too large, you can't get very close without touching things and risking mechanical damage, and the suction is pretty weak. If you think your computer needs blowing out, just get one of these:
I would start by checking the GPU fan, though. It's very common for the small fans used on graphics cards to seize up and that's a bigger problem than dust.