It bears noting that there doesn't appear to be
such a thing as a defense fleet. In the Dominion War, certain individual fleets are said to be tasked with defending specific planets at certain times - but only at certain times. In peacetime, planets in turn appear almost constantly vulnerable. Earth seldom has any starships nearby; none of the UFP planets visited by Kirk or Picard in the respective TV shows had any native defenses in evidence.
Vulcan in the otherwise rather militant STXI displayed no wreckage of indigenous ship designs after Nero's attack (but may have had one or two generic Starfleet vessels in residence, because there is that piece of ship named Mayflower
there while no ship by that name could have been included in the fleet that was launched from Earth). The same thing may well be true of "Unification pt II" where the "Vulcan defense vessels" (never identified as being part of a "fleet") would refer to a number of Starfleet ships defending Vulcan.