[D]o we assume that Starfleet had a slipstream drive project already in progress, which led to Vestas and Meerians? Clearly, Meerians were already in-service by the time of the "Pathfinder" episode, which AFAIK was the only time since "Message In A Bottle" in the Voyager TV series that Voyager communicated with Starfleet.
Additionally, is slipstream drive an actual "drive"? Or is it a form of deflector-related technology? Or both? The focusing thing on the Vesta makes me wonder, as do the modified Voyager nacelles that have been written about but not seen yet.
Slipstream is a drive, but it also requires modifications to the shielding of ships and to the ships themselves to be a workable drive--the Typhon Pact needed to acquire Dominion shield technology as a precondition for deploying slipstream drive.
As for the question of whether or not the Federation (and the Romulans, and the Breen, et cetera) had slipstream research programs before Voyager
came with its technology, I'm inclined to think that there were at least research programs in the more advanced Alpha and Beta Quadrants towards advanced propulsion methods which happened to be compatible with Species 118's slipstream.