Do a search in the help menu ... Vegas' help menu is amazing.
Otherwise, in the pan/crop window, select MASK, and then make the outline you want by placing the anchors down. Then, just key frame the movement of your anchor points, and it will move the mask with the key frames ... I know I'm not explaining it well enough for you to do much with, but to simply give you a QUICK idea & encourage you to check out examples in the HELP section.