Well, look at the reasons WHY it's vibrating... 1. Shitty tripod will probably vibrate because, well, it's a shitty tripod. 2. If you have an L lens on a t2i body... well, there you go... I see it similar to putting a drag racing motor in a VW bug... 3. If there's vibration in the body of the camera, look at why... you push the shutter button, the mirror has to then move out of the way so it can capture the image onto the sensor. Think about it, it will then induce vibration to the body. Now if you take your thought process one step further and think about how much those little tiny vibrations can affect something you are focused on at 400mm, then you can probably deduce why your pics might be 'soft'. What mirror lock-up does is exactly what it sounds like. It locks the mirror out of the way, so there's no action required to move it. que' no?