You click on some location on the timeline in the effect/gen media/track motion/pan crop screen and make adjustments. The keyframes are automatically created when you change something

Maybe what you really need is a better understanding of WHAT a keyframe really is: It is simply a representation of what the effect will look like at that point in time.

So, let's say you want to pan across an image. You would open up Pan/Crop and set the initial position (or "keyframe"). Then you click on a later position in the Pan/Crop timeline and set the next position which will automatically create a second keyframe. Now when played back, Vegas will automatically figure out each position in-between those two set positions and move the image properly.

If you simply go to the help file and search for "Keyframe", you'll find lots of information.