Is the rust evenly across the disc or is there an area where the pads were? This area can cause vibration as the pads meet it during wheel rotation. Deejay’s method to clean the disc might work, how are your pads? Maybe replacement with new would help. New pads often have an aggressive layer to aid bedding in and clean up, get a matched surface between pad and disc.
As for discs. I replaced mine on an alloy wheel last year as part of an upgrade. I think they were the original discs as no mention of renewal in the service history. They were solidly stuck to the hub due to corrosion and took a lot of Plusgas and persuasion to get off. I cleaned the hub and smeared some copper grease on the mating surfaces before fitting the new ones. I ended up taking the hubs off to do it.
