I would think the truce is because of the spread of the vamps after being bitten and they are largely uncontrolled for a while. They will go home and continue to infect, until eventually, all humans are vampires and they don't have anything to eat.
In the flashbacks, the master was always killing his victims, thus preventing them from turning I'd imagine. But, the turned in NY are just go around infecting anything they don't outright kill and spreading the worms. That can't be good for the food supply.
Also, why didn't they (after the master jumped into the sun), burn his coffin? They can't put it back together if it is ash. And, if he wasn't going to use his minions for shielding from the sun, why did he call them back? I could understand the German, but the mindless ones could have just stayed and bought him more time.