I can sort of see where they're coming from I'm afraid
Vaccinations are meant to take place with a full health check - what if something is discovered during a vaccination visit? Are the cheaper ones as competent to notice things as early? If this isn't the case, why compromise on your pet's care? If it is, then why use the other vet at all? If something is found ad vaccination, who would treat? What if the specifc vet you see for the vaccs is unaware of your multiple vett-age and notes things/makes clinical notes during the visit that may actually form part of a wider picture that is needed at a later date? What if the weights noted at the vaccination visits form part of a wider picture that is helpful in diagnosing a problem at the other vets? What if the animal has surgery or treatment at your normal place, which may have implications for when/whether they should be vaccinated?
Of course your "usual" bet could agree to provide you with cheaper vaccinations, but then they probably work on a different pricing structure overall - e.g. their ongoing drugs, operations, consults might be cheaper to balance out more expensive vaccinations, or their premises might be more expensive, or they might have more up-to-date equipment to pay for. Or, if everything is more expensive even after petrol and time to the other vets, why do you go there at all? If it's because it's better and you trust the vets more (perhaps at a routine vacc they might pick up an emerging problem earlier than the other vets?), then I'd suggest that even though it's more expensive, it's worth it for the continuity of care and the good checkover from the vets you clearly trust more to spot and deal with more complicated things. Otherwise, why go there at all?
Sorry, just playing devil's advocate really - I'm a big fan of having a trusted vet for everything and making sure that ALL my animal's notes are together in one place so that the vet examining them whether for routine vaccinations or for a problem, have all the information available to them. Vaccinations and routine stuff give part of that opportunity to build a relationship and trust with your vet that might get missed or diluted if you only ever turn up with a problem, IMO.