It sounds like they need something to distract or at least slow the major vamp and his cronies while they run over the minor vamp and make their escape. Without knowing more about your vampire hunters like what kind of weapons they use, how skilled they are, and what other items they carry it's hard to know.
Fire bomb at big bad and company, maybe with a little buring garlic smoke or something to slow them down and/or distract them for a second. Crossbow bolt to the weak vamp and run like mad.
Other possibilities:
Locked door in alley wall suddenly opens and someone motions frantically for them to enter.
They notice a door or boarded up window off the alley, kick it in and escape into the dark building.
The experienced hunter yells, "Follow me." He jumps to the top of a trash can, leaps up to grab the bottom of a fire escape ladder and climbs up the side of the building either to
roof or to an open window several stories up.
A car roars into the alley knocking vampires every direction, the door opens and someone yells, "Jump in. Hurry up."
Somone thows a toilet out of a fourth story window. It lands on the powerful vamp creating enough chaos for the good guys to escape.
A rope is thrown from a rooftop and either a rescuer descends or someone yells for the good guys to climb up.
The seemingly frightened apprentice suddenly turns into a kraken and begins eating the vampires.
A manhole cover pops up and a horde of angry, smelly ratmen grap the good guys and attack the vampires saying, "You can't have our food. We spotted them first."
Without knowing more about your story, it's almost impossible to know which scenarios are plausible and which won't work.
