Feldar I am sorry to here of your loss
I would almost guarantee that the myxo vaccine you had for you bun was given incorrectly
With the old style single vaccine half was to be administered sub q and the other INTO the skin on the ear. Many vets were unaware of this, also it needed boosting every 6 months.
HOWEVER there is now a new combined vaccine that protects against both myxomatosis and VHD that is administered sub q AND lasts for 12 months.
If you were interested in one of mine, they come already vaccinated so no waiting for the immunity to kick in
No vaccine is 100% and the old style myxo was nowhere near perfect tragically for you bun, but the new one appears to so much better