Don't sneak up to the armor crate in front of the Silver Rush Casino and steal the VanGraff Combat Armor out of it!! The game will identify it in your inventory as a quest item (because of the side quest "Birds of a Feather"), and you will be unable to drop it, give it to a companion, or sell it. You can wear it, but the Combat Armor Mark 2 that I prefer has a much better Armor Rating. I wouldn't mind as much, because there have been items in games such as Fallout 3 and Oblivion that were glitch-locked into my inventory, but at least they usually didn't weigh anything!! The stupid VanGraff armor weighs a back-breaking 25 lbs!! Unless the much-anticipated patch fixes this, my inventory for this play-through will be permanently hindered by 25 freaking lbs! (Don't really know if I'll be able to continue with this play-through anyway, due to the notorious "can't re-enter The Strip" glitch)

You must finish the quest for the Van Graffs either by giving up Cass or killing the Van Graffs. This will make the armor non-quest related and open it up for sale or otherwise. Had the same problem myself. Start the quest by speaking to Gloria.

It's not a big deal. I've been stuck with that armor 3 times through and it didnt affect me much...especially considering how much item management is required.
If you are concerned with that you should also consider:
Canine brains
Salvaged array parts
Lead (it doesn't register a weight until dropped)
Snow globes
And the array of other quest items that get locked in inventory