I have a working DraftPro 7575A. Why don't you try to divide and conquer?
First get the manual for the plotter off the internet. Second there is a key combination that lets the plotter self-test. Find the key combination in the manual. Have a pen and a D size piece of paper ready for the test. If the plotter runs its self print test then you know the internals of the plotter are OK and the problem is in the communications or the software instructions. Get an autocad print file from a friend and send it to the plotter or download a trial version of a CAD package and output something from it. If it runs, the communications parameters are fine and it will run correct HPGL. Then you are down to the output of your chosen software. Then you can call the company and get their help to work it out. Also make sure the driver for your plotter is the right one. See the internet for this too.
I hear a lot about vinyl cutting with plotters, for what purpose? Signs?