To not mess up the aspect ratio be sure the 5th icon (looks like a reverse 'L' shape ruler) is on. Also you can enter values by hand in the first icon (properties) This will ensure that you don't drift up or down as you pan. This is another of Vegas's powerful features, you can do many things with pan/crop and while easy to do once you understand the concept it may be a little confusing until you do it a few times.

A question that always comes up with pan/crop is what is the purpose of that "F" in the middle of the workspace? It is to show your current orientation. If you rotate your image that "F" with also rotate in kind as a visual cue you're no longer straight up and down or that you haven't flipped the image or have it upsidedown.

Couple tips for good panning. The span on the main TIMELINE determines the speed. Most people when shooting with a camera pan way too fast. You can easily correct that in Vegas by first isolationing the pan area as a seperate event, then while holding down the Ctrl key stretch the right edge of the event on the timeline. Then set a first and last keyframe in the FX workspace and you can smoothly pan left to right or whatever way you want. Of course you can pan and zoom in one smooth step by adjustring the size of the box with the nine handles. By setting keyframes you can begin at someone's head and slowly move down to their feet or have a close up of their face and slowly zoom out or even slowly pan a group of people, etc..