Via through TQNF Exposed Pad
First of all, one via isn't going to move a lot of heat. The reason the chip has a pad on the bottom is for thermal control; the idea is to have a good thermal (not just electrical) connection to a large area of copper (e.g., ground plane).

Secondly, vias-in-pads cause problems with reflow soldering &mdash; unless the vias are covered or plugged, they tend to wick solder away from the surface of the board, making the electrical (and thermal) connection unreliable.