Method One:

Vegas uses the terminology "event" which can be any section of a file on the timeline. You can carve up the timeline into as many "events" as you wish. The easy way (what I prefer anyhow) is click anywhere on the timline where you wish to "break" your source file into an event or a scene if you prefer that word. Click on Edit, then split.

You should see an event created by the addition of two boxes at the end of the event you just made. The top addition is the pan/crop control, the bottom is Event FX. At this point both will appear grey, these are beyond the scope of your question, so lets move on. I only mention it because it is a easy visual clue to know you've made an event on the timeline.

Place your mouse on the area of the timeline to the right of the event you just created, note how you can drag the remaining timeline contents further right or moving it to the left to overlap the event by holding down your left mouse button and dragging. Note when you overlap a blue X area is formed, this if you set other options can create automatic cross fades, drop in transitions, etc., but we're getting off topic.

Now click on the right edge of the event you created. Note the cursor should change to a double headed arrow. While holding down the Ctrl key drag your mouse to the right to lengthen the event and create slow motion. You should see a ziz zag line through the timeline. Going to the left while holding down the Ctrl key will reduce the the event on the timeline and create faster speed. Its that simple.

Method Two:

Click anywhere on a video track. Click Insert, select Video Envelope, Event Velocity. A blue line should be added across the middle of the track. Move your cursor over this line and it should change to a double headed arrow pointing up and down. Drag the line up or down to set the velocity noting the readout that appears as you change the location of blue line.

To get more control you can add set points anywhere along the line. This will allow you to move sections of the velocity envelope much more precisely. Right click on the blue line. Select 'add point' from the menu that pops up. Left click on any of the points (boxes) to move them up or down on the line to control the envelope.