You're assuming the'res some inherent logic to Cryptic's ship vetting process. If we take your assumptions about Cryptic to be true, we would have seen the Ambassador long before any news about the Vesta popped up. The Ambassador is canon; the vesta is not. The fact that the vesta is going to be in game before the Ambassador is making me laugh.
And the same could be said for just about every cannon ship that could have already been added-Andorian battle cruiser, New Orleans class cruiser, Kelvin type cruiser, etc.
The Ambassador was listed as a possability over a year ago, along with the Vesta. Development stopped on a lot of things around here because of the sale to PWE. Now that the game is strictly FTP, they have more incentive to begin churning out ships the players want.