I'm going to be a little nit-picky here. You can't CONVERT an electric toilet to a VacuFlush. But you can remove an existing toilet, either manual or electric, and replace it with a VacuFlush system.
Keep in mind that it doesn't only consist of the actual toilet pedestal that mounts in the head; it also requires a vacuum pump and vacuum tank that must be mounted below deck somewhere (between the toilet and the holding tank). Also, SeaLand requires that a VacuFlush system be installed by their people - or if you install it yourself, the installation must be inspected and OKayed by their people, or they won't extend warranty coverage on it.
The $2K figure for the parts is pretty close. Installation can run you about half that much more. I don't know about labor rates in the "heartland," but along both coasts a marine plumber gets in the neighborhood of $100/hr., sometimes more.