*grins* Can never resist a good arguement.
1) If a branch of the US goverment could survive, why couldnt an arm of the church? Mainly the inquisition. Sure robots could be more lethal then vampires, but if you believe vampires are Satan's spawn, which would you focus on?
2) A communcations network does exist. Remember Threedog. He gets his information from somewhere, even if it is from a runner.
3) By the admission of the very folks in Canterberry(sp?) not a single person was killed. Comparing that to a person who drinks your blood and bursts into flame in direct sunlight (not to mention some of the 'clans' actually require eating of flesh too)..
Now for a somewhat reasonable tie in for fallout, what if the vampires were in positions of power before the great war. If some goverment got wind of the vampires existance and was about to organize a campiagn against them, how would they react? My money is they'd push the button and trigger the great war. *takes off tin-foil hat*
You make valid points, but I must retort:
1) Perhaps an inquisition would work, but there is no strong presence of religion in fallout. The nuts in megaton are much more interested in "becoming one with atom" than to do anything useful, and that little church in rivet city appears to be the only functioning church in the area, quite probably the entire state. Perhaps say, the Regulators would make for a better foe for vampires.
2) But Three Dog is all there is, and most people dont appear to put too much stock into what he says. With all the ridiculous and out there stuff he says most people would probably hear him say "vampire" and think to themselves "yeah whatever, three dog".
3) But death hardly phases most people in fallout, hell that kid in greyditch doesnt pause for a second when you tell him his dad is dead. It wouldnt matter if the antagonizer killed a hundred people, I wouldn't expect many wastelanders to care either way. Indeed most people are pretty jaded in fallout, I'd imagine most of them are vaguely aware of strange meat's origins and I'd bet they're not so much interested in the "good fight" other than its something other than enclave radio adresses.
Infact I'd bet you could walk right up to somebody in megaton, bare your fangs and say "raaa! imma vampire!" and they'd be like "yeah whatever, i got stuff to do - no time for this crap"
ETA: Also if you need an anti-vampire weapon, why not just use the flame thrower? Don't need no silver bullets or purified stakes, just burn 'em. I'd imagine thats how a real world government would deal with a hostile vampiric threat.