No way Ranger Vets are better trained; Paladins live in an environment that raises them for war literally from childhood. They are a warrior caste, and their sole purpose is to fight. Nothing else. They are raised for this their entire lives. If you're a Ranger, you enlist in the NCR military at maybe 17 or 18, so a Paladin's already got years of training and conditioning on you. You go through crappy basic recruit training, spend some time as a grunt, and then finally get appointed to the Rangers. Meanwhile, the Paladin continues to receive the best possible training this whole time.
It's the same reason (cliche, I know, but it's the best anology) that the Spartans were the best Greek fighters. They trained for it their whole lives, from birth, to the exclusion of all else. Same goes for Paladins.
Ranger Vets may have more experience than the new Paladins that have been cooped up in the Hidden Valley complex for years, but better training? Not a chance.
Where the hell are you guys getting this Paladins are trained from birth [censored]?
"After many years of service and experience, the best Knights are promoted to Paladins"
A Brotherhood Paladin goes through a similar ranking system as a Ranger Vet
Paladin= Initiate-> Apprentice->Knight->Paladin
Ranger Vet= Recruit->Grunt->Ranger->Ranger Vet
Okay, even still, from a closer ambush position, I'm very certain a Ranger Vet could easily one shot a Paladin through the neck. It would even be easier for the Ranger Vet because the closer range would mean an easier snipe. And if the Vet misses? Well he wouldn't have been a real vet anyway.
A Ranger Vet could very easily ambush and kill a Paladin.