The place that does the medial for London embassy has a price list for their vaccines, here they're about same price as private, some more, some less. See if you can find yours? Can maybe email them and request to if you want?
You can either get them before, if you can get any free may as well get em (doesn't matter if second time), get them at medical
or you can get them in US but lord knows what cost is there off insurance. You won't need them until adjustment of status I think? Don't think K1 requires any vaccinations, would just mean a second medical in US after you have vaccinations.