Stretch will make your picture look strange. Its just a fact of life with HD.
I had comcast HD STB and it did same thing. I could choose Stretch mode and have wierd looking fish bowl picture or use 1080I and let it downconvert and have that box picture with black bars on all sides. and picture is in correct dimension. Did not bother me.
With VOOM there is an easy fix and it just requires toggling your video button on your remote control.
What i do is that I have my Standard Rca jack output to my home theatre system and also my component HD also thru my home theatre system.
My home theatre receiver outputs my standard RCA signal to Video 1 of my Sony Wega, Then my component HD goes to input 2 and my DVI is plugged directly into input 3 of wega.
No when I am wathing a HD channel on my Voom box i toggle my Tv's input to either input 2 or 3 for my Component or DVI signal.
Then when I am watching a SD channel on my voom box I switch my TV to input 1 (rca jack signal) and I get normal 480I full screen picture that is not stretched. Just like I would with Dish or Direct or Cable.
Heck even my wife can do it so I know it is not that big a problem.
Other then that I dont know your paticular set but if you have Voom box set to 780 or 1080 and
dont want full screen I think your going to be unhappy unless your stretch works better then mine.
Just tryin to help.