^ Remember, Robocop had a model of Vengeance sitting out on his desk for all to see. What did he do when other officers or officials asked about it? I'm surprised there weren't rumors all through the fleet about it. Unless Enterprise was the only ship that never got any of the gossip?..
Entirely possible that the Vengeance's design wasn't secret, just sold as being something other that what Marcus ended up making.
Also - I thought that maybe that lineup on the desk represents key starship design evolution - and that perhaps the Vengeance was there as a symbolic nod to the future of starfleets direction (ie: a model of the next generation of Starfleets ships) - not necesarily of what was understood to be current designs (after all, starship creation could take a long time from conception to fruition - and perhaps the Vengeance was being advanced at a much faster rate than was anticipated by the general public...)
I liken it to a display of car models on a desk, all being significant milestones in the motoring history, with a model of some kind of futuristic car design (not yet made) but at least designed on paper.