I suppose, depending on the playback device and the television. Most DVD players have the ability to specify whether they are connected to a 4:3 or a 16:9 set. If you tell the player it's a 4:3 TV and put a standard 16:9 widescreen video in it, the DVD player will letter box it for you automatically without you ever having gone through Vegas at all.
I guess it might help if you started at the very beginning and filled in the blanks: i have a ______ video on a ______ (disk/media) that i'm going to play in _______ device to watch on a _______ screen. Without this information we're pretty much only guessing what it is you're trying to accomplish.
Leave "maintain aspect ratio" checked. You want this on.
In Pan/Crop you can choose either the 4:3 preset, or right-mouse-button click in the cropping frame and choose 'match output aspect'. Note that this will cut part of the left & right sides off of the image, which is presumably what you want to do based on your other comments.
As others have suggested, why don't you spend an hour or two trying various things and document your trials and your results. This would benefit you better than spending the same amount of time asking multiple questions in your post. I experiment a lot and I think that you gain a lot more experience this way. By all means, if you are stuck with anything and can't seem to make any headway, ask away and you will get help from very patient, dedicated forum contributors.
There's no substitute for diving in and getting your hands dirty. Just make sure you document everything for future reference.