If you are using Vegas 4, hit 7 (goes to start of selected event) or 9 (goes to end of selected event) on the numeric keypad to activate trim mode.

Then use 4 & 6 to trim by 1-pixel (zoom in for more presicion) or 1&3 to trim by frames. Hit 8 or 2 to exit trim mode.

Holding Ctrl or Alt while using the trim keys will have the same effect as when editing with the mouse.