OR... well, honestly, does ANYONE know just how much time passed between (a) the Mutara Nebula incident, (b) The return of the Enterprise to Earth, (c) the flight to Genesis (where the Enterprise was lost), (d), the stay on Vulcan and the return to Eaerth, (e) the trial, and (f) the flight through Spacedock where we see the 1701-A revealed?
According to Memory Alpha, the events of Search for Spock occur in 2285, Voyage Home is 2286. I'm not factoring in Wrath of Khan because I find it difficult to believe that they would start building a new ship after hearing of the damage from the battle with Khan, then lie about it to Kirk and Co. That strikes me as extremely unprofessional and simply petty. Keep in mind they had just gone through a horrible battle and Kirk had just lost his best friend. Now would the Admiral, knowing this, then decide to give Kirk more bad news just so he could 'surprise' him at the end?
"Yeah, sorry you lost your best friend and had to watch a bunch of trainees die in a battle of personal revenge between you and Khan. Oh, and we're going to decommission the Enterprise as a joke."
Now, 2285 -2286, that doesn't really give us much of a time frame. They may have had a whole year between the destruction of the Enterprise and the big reveal at the end of VH. The events of SS take place pretty quickly. As soon as Kirk knows he has to go back to Genesis, he immediately starts on it. And it appears that Saavik, David and Spock only spent a couple of days on Genesis (that is if Genesis has Earth standard 24 hour days). Spock was aging pretty rapidly, so not a lot of time could have passed.
At the beginning of VH Kirk says in his captain's log that they've spent about three months on Vulcan. Vulcan is a little over 16 light years away from Earth. I'm not sure how fast the Bounty was travelling, we could give them about a week or so, if they were rushing. Then the events of VH pretty much happen in no time at all, Kirk goes back in time then forward in an instant.
So VH gives us about four months, we can be generous and say five or six (we can add another month between the return from the past and the trial). I'm not sure how much time to give SS, maybe six months, but I'm not inclined to think that 18 months passed between the two movies. I tend to think that they did simply refit and rename the Yorktown, because there simply wasn't enough time to build an entirely new ship.
Or we can believe that they did the impossible and managed to build an entirely new ship in such a short time span, a rush job, and that was why Enterprise A was in such bad shape for FF.