Velocity Envelopes can be used to speed up, slow-down, and freeze video in Sony Vegas. To use a velocity envelope, right-click a video event and choose Insert/Remove Envelope > Velocity. Drag the envelope up to increase the clip speed or down to decrease the speed. Drag it to 0% and the clip will freeze. Setting it to a negative value will cause the clip to play in reverse. Double-click to add anchor points to speed or slow down the clip at different points. Remember, if you speed up or slow down a clip with velocity envelopes, the clip will vary in length and may need adjustments to avoid looping.
Tip: While dragging the velocity envelope hold the Ctrl key to move it in fine increments.