She said she was going to have to owe you a really big favour.
Neil Gaiman, Neverwhere
The Prestation system got a bit overly formalised for me, pretty much right away. Boons were scaled and measured, and the idea of Harpies keeping a tally of debts and credits has always seemed a bit left-field to me. Someone keeping track of such things, sure, but why the gossips who determine status? Because the loss of status is the stick that encourages repayment. Makes sense, but I’d be tempted to move it to an imprecise system and keep it off the books. (On the books is good for LARPs, where the lines of communication are less reliable - if the ST at the tabletop says the harpies all know and your Status drops, that’s it, but a LARP relies on several players sharing that information and comparing notes will make that easier.)
(“The Equalizer has the best New Wave Requiem credits sequence ever made that doesn’t have Bauhaus in it.” Discuss...)
This leaves plenty of room for other ways for vampires to work together, and more importantly, to screw each other over, while trying to maintain a reputation for keeping their word and paying their dues.
And yes, the younger and less powerful Kindred are more likely to be screwed over by their elders due to their lack of strength in negotiations. This is part of the structure of the setting. (Go Anarchs!)
Of course, the elders can’t betray the PCs every time - even Shadowrun sometimes has Mister Johnson skip the part where he betrays the runners and provides a final boss fight at the end of the adventure. Reputations can be important, so the Harpies’ influence over the web of favours comes back in, and rightly so. But always be looking for an angle.
And then there are the things a vampire is simply expected to do. Don’t ask for payment for upholding the Masquerade, or observe how many tasks seem to be counted as upholding the Masquerade...