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.