I’ve used vinyl Matt for a white wash mist

The best paint to apply as a first mist coat would be paint for new plaster or a contract matt. These do not form a paint film as vinyl does. But you say it was watered down? If it was then primarily it will have been absorbed by the plaster which is the aim. So you should be ok

Lightly rub down the walls with 120 grade sandpaper apply your finishing coats

so long as you have applied a mist coat using matt emulsion and diluted around 50% then you shouldn`t have any problems. The shiny patches are more than likely just where the plasterer has over troweled the plaster causing a shine on the face. A light rub down on all surfaces to de-nib and form a key prior to your next coat of paint should be perfectly fine.

For a mist coat it is best to use watered down contact mat emulsion and not vinyl mat , that’s probably why there are shiny patches , just carry on now and apply normal emulsion ( not watered down)