Pan and Crop is a good way, but almost always leaves you with 3X4 size window. Try applying the cookie cutter to each window. (You'll have to experiment to get the results you want, but I've done it. I've only done it a couple of times and had to rack my memory...no, I had to figure it out again, the second time too.) I don't do it enough to remember the steps, but if you mess around with the cookie cutter you can change the rectangle ratio. (Or even make bizarre shapes for your window.) I don't know that you will be able to make each window fit into a perfect 3X4 'puzzle' for your TV screen, but you can use a 'color' backdrop and add the windows on top of the backdrop to speed things up. Each effect added will increase rendering time, in case you don't know. Also, whichever screen is on the top of the 'timeline' will cover any screen under it, so you can overlap a screen too. If you experiment with the cookie cutter, play with all the options it has to offer, you can see immediate results in the preview window.

It varies depending on which version of Vegas you have but regardless, at least from version 3 on there is a button to either lock or unlock aspect ratio you can drag the pan/crop bounding box as you wish if that's what you want to do. Once you got the rough size you want right click on the image within the pan/crop work area and select 'match output aspect' and it will jump out to fill the frame filling the bounding box with the area within it. You then can tweak if needed.

There also are buttons to lock movent in either X (sideways) or Y (up/down) so when you drag the bounding box around it only move in one direction. Vegas is very flexable. You can lock the aspect ratio, move only in one direction or do free style where you totally ignore aspect ratio and stretch or rotate the image as you wish.