I'd be inclined to save up and do them all or at least a group in at the same time, you will often get a multi-bun discount if you take a certain number together.
Regardless of the infection, there is no way a vaccination should be given at the same time as a surgical procedure. The bun's body will have been challenged by the anaesthetic and the bun's body and immune system will be concentrating on repairing the wound and fighting infection in the wound - hardly a good time to give a vaccination. If the bun's body is busy doing those other things, there's a good chance that sufficient immunity will not develop, and/or make the bun poorly as its body struggles to deal with everything thrown at it in one go.
Being near water increases the risk because biting insects like mosquitos and gnats come from the water - the larvae are water-borne. So being near water means you are likely to be near a larger source of insects and therefore an increased likelihood of infected insects.
Mine are done 6 monthly; we have wild bunnies nearby, a major river just over the road and both neighbours have ponds, so I always make sure mine are done like clockwork at 6 months on the dot as long as they are well enough.
Well mine have been done 6 monthly for years and I've never had a problem. But if a bun has an underlying health condition which has not become apparent yet, it is possible that giving the vaccination will cause their immune system to become 'overloaded' and make them ill. It's pretty unpredictable though, but uncommon