Personally, my advice is: Don't.
Save NPC Advantage for social situations, where it can stretch its legs and make it clear what the game is about. Advantage/Disadvantage just steps on the toes of Soft Hits and Hard Moves in combat. I'll talk about this more lower down.
In the case of your example, here's the thing, as far as I feel:
Outnumber isn't sexy enough to be a condition. It doesn't mechanically strengthen the vampire gang in a way to be immediately relevant.
As soon as they get the String from the failed Hold Steady? Immediately spend it to come out of nowhere with a Hard Move. The PC is outnumbered? Make your Hard Move prove it in the fiction.
"Okay, so there's a bunch of 'em, and you're spooked -- your failed Hold Steady proves it -- maybe you try and bolt, but c'mon there's loads of 'em, you bolt right into one standing in your blind spot. He gets you by one arm and another grabs the other and now you're pinned against the dumpster. The one in charge -- the one with the stupid haircut out of the '50s -- he's moving in now. What do you do?" (put them in a spot)
So, let's break it down further. What does the PC do? Well, let's say they attack -- call them out on that. Okay, how? Your arms are pinned and you're against a dumpster. Okay, you're flailing your legs? Trying to hit 'em in the shins? Now, make the call: Is screaming and flailing actually Lashing Out? Let's say you judge, yes, that's totally Lashing Out. How many can the PC hit?
This is really, as far as I can tell, the meat of the question. How do you make a gang fight not a one-off or a series of one-offs. You do it by the letting the characters work over more than one baddie at a time (I personally would limit how many times I let a PC rip into groups... they're just kids, after all). Let the PC hit a couple of 'em if she succeeds. Boom Boom, problem solved, she's kicking her captors in the shins and that really smarts even if you're a blood-sucker and now it isn't one-on-one cause it just became one-on-two-or-three. If you want to keep it one on Many, challenge the PC to be smart about how she is fighting.
Now, let's talk Advantage/Disadvantage like I said I would. You give the PC the Condition "Outnumbered" (not one I would do myself, only because I've explained to the players at my table that they should think of most conditions as the psychological or social ramifications of self-labelling). Okay, NPCs act at Advantage when using a PC's Conditions. What can they do: gain new followers, be better protected, perfectly follow up.
Here's my issue with Advantage/Disadvantage in combat. Those all sound like Hard Moves. They sound like Hard Moves that you can make just by describing them, not by earning them on PC's Misses. So, how to use "Outnumbered" as an Advantage in combat...
Boom, she fails a Hold Steady Roll, you spend the String, Outnumbered. You describe (because she just failed, so you're talking) the Vampires swarming around her like some sort of crappy broadway dance number, and then they're on her and fangs are breaking skin. Or whatever. Because they're perfectly following up which is one of the things you can do with Advantage.
So, as long as that Condition exists, you just keep having them perfectly follow up. It can make group fights really deadly, which makes sense in a way. I don't like that though, cause again I don't like the way Advantages translate to combat.
So, I guess what I'm really saying is that a fight of one versus many can be made to feel like one versus many by playing to the fiction. If the PC leaves themselves open to retaliation from the many, then hit them with the harm from the many. If they have a way of thrashing on more than one at a time, let them have it. If they have the condition Outnumbered and they successfully lead the many into a warehouse and fights them one by one, lose the condition Outnumbered.
Just my musings on the idea. I've had plenty of one-versus-many fights, and its about remembering your angles, at least when I'm running them. A Soft Hit is a great opportunity to remind them you can't half-ass it when you're surrounded, you'll get beaten down also. PCs need to control the situation when outnumbered, and so they have to watch the stakes they're fighting for and control the battlefield.