Prosoft is the only vendor that A-B has licensed to build interface cards for the 1756 ControlLogix platform. All of the MVI56 modules, including the "MVI" that A-B sells, are the same engine under the hood.
If you don't want to buy Prosoft because of price, then you're barking up the wrong tree. If you don't want to buy Prosoft because of the configuration method, then you can just get another Logix CPU and put it in the chassis. They support all the DF1 protocols out of the serial port.
I've heard about A-B planning a DF1 card that "bridges" messages to a CPU across the backplane instead of using the database method that Prosoft uses, but I don't think that will be released in 2004.