I would like to hear from anyone that has experience dying vinyl interior parts. I have been looking for a black, late model A-body buddy seat for quite some time. I am going to keep looking but since I haven't had any luck so far I bought a burgundy one to dye black for the time being. I would like to know what the best product is for doing a color change and also what prep is necessary. I would like to do an actual dye of the vinyl so the color penetrates into it but looking for vinyl dyes it seems most are more like a spray paint.

SEM Color Coat: Link

I have used SEM on dash pads and various plastic bits. It looks like brand new oem type quality if done correctly, many light coats and it will lay out like it was made in that color.

I know Herb McCandles also makes aerosols that match the various factory colors including metallics for interior plastic parts.

I have used SEM on various interior pieces but only in areas that are not used continuously like a seat would be. IE: Upper door pads, Visors etc. The buddy seat might depend on how much one uses it for an armrest. I would call SEM and ask them about it and whether heating to vinyl to a given temp may help .
Rustloem has recently introduced a vinyl fabric spray that I've only used once. But I was quite impressed with how well it looked and adhered to a dash pad.

One of my oldest friend's owns a body shop. He and I have always used an adhesion promoter first

It softens the plastic or vinyl or vinyl top so that the dye has a true chemical bond when applied.
I have used PPG and DuPont that were mixed to color and sheen from a spray gun and spray bombs. And SEM premixed and bombs. I do not know if SEM mixes cors shades like the paint jobbers do with DuPont and PPG do. Used as a color change not to repair fill old looking plasuc or vinyl. IE don't think you can spray tons on an old crap dried out missing texture plasic panel and make it new
If cleaned right and promoter used the dye was fine on vinyl tops in the hot and wet south. And many years later was fine. I did see where a driver seat wore through. But was used plenty for years and years.
Tops always looked like new when done and saved big dollars over putting a new one on when color of paint on car was done.
You can get Bull Dog adhesion promoter in spray bombs if that is what you are doing Premixed Cans.
My peference is to use a gun. But cans are ok.

SEM makes clear coats as well. For a high wear item like a seat or armrest, I think I'd spray a couple coats of clear over it for a little added protection.

As I understand it, technically they are all actually paints, there is no such thing as actual vinyl dye even though that description is used. I thought long and hard about it when I wanted to dye (paint) the rear curtain of my convertible top. After reading a lot, SEM always came out well, but the biggest takeaway was surface prep and the adhesion promotor. Do that right and I think most of the available products will work well. Something to soften the material before spraying it.

I haven't used SEM dyes but man their interior sprays are great! I've done dashes, and even steering wheels with a spray and they come out good and even the steering wheel held up really well!

I always clean the Vinyl or Plastic with Acetone or some other high strength solvent that will not leave a residue.

It will "soften" the surface and helps the paint/dye to sink in better. I died a set of White seats in my Challenger black. Even as a daily driver with lots of use the dye stuck pretty well. Only when scratched with something hard and sharp, did the white show through. Normal wear and tear had no problems and they stayed black even on the seat bottoms for years.

Clean it right before you paint them so the surface is still soft when the paint is applied.

My

Auto body paint supply mixed up a quart of SEM for me, and I sprayed a pair of green '68 Charger seats white, and they held up great. Couldn't tell they were sprayed, even in the pleats. Just wiping with a solvent just dilutes contaminates like Armor All, or whatever. I use trisodium phosphate, which is a powder, with a Scotch Brite and water to scrub them, and wash all contaminates off.

I have used SEM vinyl dye on things like dashes, hard plastic trim, and vinyl tops and it works well. I have not seen it work well on vinyl or leather seats though. For something the size of that buddy seat why not just get it recovered in black?