i'm playing as a stealthed Sniper, and plan on using the Varmint Rifle as my primary weapon. what mods would be best to use? i walk around at night so i'm already going to buy a Night Scope, silencer, but what about my 3rd?

thanks in advance

There are only 3 mods total for Varmint, the 2 you mentioned and the extended mags.
But, it really isn't worth upgrading the varmint, just get the unique one instead (Ratslayer). It has all the varmint mods pre-attached, does more damage and DPS, and has a whopping x5 crit modifier (normal varmint is only 1x crit modifier).

oh alright, thanks. i didn't know it had the mods pre-attached

If you use VATS, then I suggest upgrading the Varmint Rifle and forgetting about Ratslayer. Varmint Rifle + VATS = 2-3 shots; Ratslayer + VATS = 1 shot.

If you DON'T use VATS, then, yeah, Ratslayer is kind of awesome.

And keep your regular Varmint Rifles to repair it.

Be warned. You might die a few times trying to get Ratslayer. The critters guarding it are like micro-deathclaws that can kill a sub-level 10 character in 3-4 strikes. If you can get all three mods for the Varmint Rifle that'll be good enough. Only giving up a few points of damage in the end.

yeah. i found that out. i used a stealth boy and a silenced 22 on the first 2 rats in the room, then tossed a grenade down below with all the other rats. then i waited until they started running up to me, jumped down, and ran to the exit without taking any damage. i don't use VATS that often so my new found Ratslayer is definately paying off.