As the title says, Blake in the Crimson Caravan sells armor that will often disappear from your inventory after it's sold.

I was already wearing combat armor and when I bought a new one to use it to repair, it disappeared from my inventory but still took the caps.
Same thing often, but not always happens with recon armor.

I am having the same problem with Blke and Armor purchase thru Servatron Gun Runner and Mick.

A quick fix for this was to drop the armor I was wearing and only have one piece of that armor that you want in your inventory at a time.

Pain, though. Hope it gets fixed.

For some reason it seems that vendors can carry items with the same unique object ID value. This is how the game normally tells one anti-materiel rifle from another despite having been both repaired to the same quality number. I've noticed this glitch more with weapons than armor- but yeah if you drop one of them from your inventory- miraculously the other will now be visible. Until (if it ever is even) this is patched, just drop one item on the ground and sell the other when you find items that do this. Either that or put one of them in a follower's inventory.

Remember, if more than one of these mis-ID'd items is in the same container it will hide one of them from you.

Also be warry of hanging on to duplicates by putting them in followers inventory or in storage containers. This COULD be one of the causes of save-game corruption thougth this is not likely (since I have savegames with items like this that are not corrupt- just don't tempt fate.)