Could anyone tell me what weapon mods are called? Are they special titled at all?

I have cleared startertown, Primm, and Nepton, and I still have yet to run into one. Not even one for sell... I kinda figured the guy who told me about them would at least sell one to show you how it worked, but nope...

Am I just not far enough yet?

They are on the MISC tab for vendors.
Chet the guy in goodspring has mods for the starter type guns. If he doesnt have what you want wait for 2 days then look again, stock seems to change for those every 48 hours or so.
Almost every town vendor in game has them from what Ive seen,

Yeah, Chet has some now and then; I remember seeing a Night Scope mod thing for the Varmint Rifle once.

yeah he has a few ive seen some but the only one i bought was the scope for the varmant i love that gun now

So far from chet i've seen: scope for 9mm,Laser rifle focus optics, cylinder for 357, extender mag for 9mm,extender mag for varmit rifle.

Does anyone know how often these guys get new inventory? Mine seems to have been stuck at the same stuff for a few real time hours (around 6), and none of the vendors have updated their stock when I played that block yesterday. Multiple in game days passed.

I bought exactly that from Chet. I guess I'm gonna slightly thread hijack now... slightly.

I have a Varmint Rifle, and I have a Varmint Rifle Night Scope. I've tried equipping the Rifle, and then going to my Misc tab and clicking the Night Scope... I've tried not equipping the rifle, but clicking the scope...

I've tried right-clicking the scope in my inventory, which just makes me drop it.

What am I missing? How do I equip this thing? I thought it might be like the WMK mod for F3, where you need to use a workbench, but... no... you don't get a new recipe when you have both in your inventory or anything. What do I do to equip it?

EDIT: If it's anything like Fallout 3/Oblivion, a key ingredient to making vendor inventories respawn is to not be anywhere near the vendor or the vendor's cell. If you're there, things don't seem to refresh. Hey, on topic!

On the "weapons" screen in your pip boy, when any weapon is selected, there are now two choices: repair or mod. Mod will be greyed out most of the time unless you have an unused weapon upgrade for that weapon. Default key is X on the PC.

I.... am an idiot. This is an excellent example of me reading the manual, googling, using a forum, and forgetting to look at the SCREEN.

Heh, thanks a bunch.

I found some very good mods from the vendor in The Fort, a laser rifle ammo regenerator? Yes.