So I'm wanting to build a run n' gun build with Vance's SMG, but that thing is a flippin' hog when it comes to ammo consumption. Anyone know of a good source of enormous quantities of 9 mil?

Most merchants sell 9mm in large quantities so that would be your best bet

Most merchants? I need the NINE MIL MEGA MERCHANT!

NCR merchant at the Dam has the largest quantities of ammo. After that? Probably the Great Khan armorer or the Gun Runners.

I'd say just make a run through each restocking merchant.

Yup. Hoover Dam Quartermaster, Great Khan Armorer, the Gun Runner Shack, and Alexander under the 188 overpass. I use Vance's 9mm SMG all the time. Great weapon, especially against Cazadors. Each of those guys usually carries lots of 9mm.

Take Trigger Discipline, to cut down on ammo consumption and increase hit probability.

Doesn't Contreras sell a [censored]load of 9mm? Or is that mostly 12.7 and .308?

Black mountain in the 3 buildings in the storage crates. There's tons of 9mm pistols with it as well

It's mostly 5mm surplus, 12.7mm and .308 from my experience.