A thousand times this, but not where my Courier can't access, just where I'm lining up a shot and I hit nothing when I fire - the nothing right in front of the enemy that should be dead.

Edit: By the way, go for vote for Fallout if you haven't yet. By clicking http://www.g4tv.com/deathmatch/best-franchises-2011/battle/88/assassins-creed-vs-fallout/. =D

I get extremely annoyed by the vendors. They either seem to:

a] Replenish their stock/caps very slowly.
b] Never have the mod/ammo you are after (even if you go to every single damn one).
c] Not have more caps available, even when you buy something expensive; like the implants at the Medical Clinic, if I pay 12,000 caps then they should then have 12,000 more caps so I can trade.
d] Never stock a particular type of weapon/ammo while having loads of others. How can a place have 80 missiles but no bloody sniper ammunition?!

That is annoying. I tend to just buy up and horde ammo in Novac though, so never had a shortage when I've played.

I noticed the weapon mods in a merchants inventory changed when I loaded up a new save once. So I'm guessing they change every time you load. Try quick saving, quit to menu, reload save, and check the vendor again. S'immersion breaking, I reckon, but if you're desperate for a weapon mod... me thinks it'll work. ^_^