Vinyl matt will form a skin over new plaster as opposed to contract matt which will soak in and bond. It's unlikely to fall off, but I've had experiences where decorating some years later, paint has come away from the surface onto the paint roller. If it was sufficiently diluted, the chances are it has soaked in to a degree and won't be a problem. I wouldn't lose too much sleep over it, just bear it in mind for next time.
Thanks will remember for next time! The mist coat was very thin so hopefully will be ok. I'm getting on a bit now so the next time it gets decorated it will be by a pro so they will have to sort the problem if there is one! So long as it doesn't fall of in the meantime!! Cheers
I know painters that bang on contract matt not thinned over plaster ect. Common practice. They don't seem to get problems or even care. 2 coats job done. I wouldn't though as sounds crazy