Any vaccine will carry risks, and an older rabbit may have a harder time with it, depending on the rabbits overall health. But the benefits far outweigh the risks in my opinion, especially if rhd2 is active in your area. Particularly for an outdoor rabbit that has a higher risk of becoming exposed. Even an indoor rabbit can be susceptible as rhd can be tracked in on shoes, hands, cloths, by insects and other pests. So indoor rabbits are at risk as well. As for effectiveness of the vaccine, the study I read showed it was entirely effective in protecting the vaccinated rabbits.
https://rabbitwelfare.co.uk/rabbit-health/disease/rabbit-vhd/