If host A and B are both vaccinated, then either the virus has achieved vaccine escape (it is spreading amongst the vaccinated population effectively), or if it hasn't then it still dies out.
If 1 vaccinated person infects another say 3, but then those people fail to infect anyone else - then it doesn't actually matter what selective advantages that viral strain had - 100% of the virus has died out.
But if the same chain terminates in the vaccinated people then infecting an unvaccinated person...then the new mutation escapes and potentially becomes dominant within the reservoir (unvaccinated people) and a failure to spread through the vaccinated population doesn't lead to the destruction of the mutant strain.
If 1 vaccinated person infects another say 3, but then those people fail to infect anyone else - then it doesn't actually matter what selective advantages that viral strain had - 100% of the virus has died out.
But if the same chain terminates in the vaccinated people then infecting an unvaccinated person...then the new mutation escapes and potentially becomes dominant within the reservoir (unvaccinated people) and a failure to spread through the vaccinated population doesn't lead to the destruction of the mutant strain.