The reason allied soldiers can still fight and kill them is because after there first encounter with these "super" soldiers the Americans invented special ammunition to kill these vampires but it only kills them if it pierces their heart too bad they cant shoot for shit
Also researchers found out that the religious dedication Americans have for freedom, oil and everything thats American weakens the vampires so technically the Americans did win the war as without them the vampires wouldve been able to fight at full power eventually

So anyways the reason the vampires are in the game is because they found some in real life and the vampires hypnotized them to add them (the vampires) into the game as Obersoldaten, notice how if you google Obersoldaten that you wont find anything other than the ingame unit thats because the vampires had to be under a guise so that it wont raise any suspicion
Now most the Nazi vampires or Obersoldaten as relic calls them are on the run because if you're a Nazi thats bad if you're a vampire thats bad if you're a Nazi vampire THATS VERY BAD