If they make .50BMG as rare as .45-70 before patch it wouldn't be too bad at over 300 damage...but I would say no critical damage just so that things wouldn't get out of hand.
I wouldn't say
that rare, as finding more than about 6 rounds at a vendor was a minor miracle, but after extensive testing I would say you could safely divide the current allotment by 10 and still have enough for it to actually be worth using the weapon.
I am taking into account the possibility that someone will do as I do and constantly recycle the vendors to get more, because if you do this it is rather easy to end up with thousands of .50Match after conversion at a reloading bench. In fact, dividing the current allotment by 15 might even work, however I am also assuming one has unlocked the Khans' Armorer and the Hoover Dam quartermaster; if this is not the case then it's a bit harder to get large quantities of .50s without heavily recycling the Vendortron's inventory.
My concern about making the ammo too rare is based on the current situation with the Fat Man: even with my tweaks to make explosives weapons as deadly as I feel they're supposed to be I still almost never use it, since there's only about 15 rounds in the entire game. Granted there are a lot more .50s in the game than that even without vendor recycling, but the point holds.
Definitely agreed on the no critical damage if the base is that high, since that's already enough to one-shot pretty much anything less than a Deathclaw Alpha or Mother, and if a head-shot probably even those.