Rabbit vaccinations depend on where you live. In the USA, they don’t recommend any vaccines as the wild cottontail rabbits don’t pass on the disease so it doesn’t spread around and infect domestic rabbits. Although there have been the odd outbreak or two at times.
In Europe and UK, rabbits should be vaccinated for both Myxomatosis, and RHDV, including the new vaccine for RHDV2 which the old vaccine doesn’t cover. I’m not sure if the new vaccine also protects against the older virus. There are a couple of different vaccines for RHDV, a combined one with myxo, and also a couple different variations, I can’t remember the names of but I think some are more likely to produce reactions than others.
In Australia, the myxomatosis vaccine is not available, neither is the new RHDV2 vaccine, however rabbits should be vaccinated for RHDV with the cylap vaccine (the only one available here). Currently in Australia, both myxomatosis and RHDV2 are spreading rapidly and are killing many domestic rabbits. Our wet winter and mild spring has led to a boom in mosquitoes and flies and encouraged the spread of both diseases. Both are terrible and should be avoided at all costs. If your rabbit gets either, it’s almost certain death. Myxomatosis is a terrible disease that the rabbit suffers through, vets recommendation euthanaisia as survival rate is very poor, and rabbits with the disease can only be given supportive care during which they can suffer quite immensly. Calicivirus is more sudden acting, the older strain tends to have symptoms, the new strain tends to have no symptoms at all, a rabbit can be fine one hour, and the next simply dead. If you live in Europe, UK, or Aus, definitely give whatever vaccine you can, especially if you keep your rabbits outdoors. My pair are indoors, but I still vaccinate them.
Flies/mosquitoes can still get in the house, and RHDV/2 is quite stable on the environment and can be brought in on shoes/hay etc.
Edit: I forgot to say, the only downsides to vaccinating that I know of are the possible side-effects from the RHDV vaccine. The vaccine needs to be rubbed in thoroughly for several minutes after injection and if not reactions are often more likely. Side effects include fur loss and scabbing near injection site, in-appetite, lethargy, and sometimes fore-limb lameness which usually goes away in 24-48 hours. The only reaction I’ve ever seen in my buns is a bit of lethargy for the rest of the day.