Paint Vinyl onto Supermatt Emulsion

Yes it shouldn't be a problem. Just make sure the surface is clean, dry, dust-free etc as you normally would, and give it a sand with some light grade sandpaper to key it a little just to make sure adhesion is as good as can be. Generally speaking, emulsion over emulsion should be OK though unless there are any underlying problems with the surface eg. damp / moisture, in which case vinyl emulsion or any water-proof paint will not be a good idea as it will not allow the moisture to escape and you will get large bubbles appearing. This is a pain to fix and is best avoided at all costs.

Yes,any emulsion on emulsion is fine.silk on Matt would never be a problem and you would need to do at least a minimum of two coats as with all paint so as to get the true colour.For the area you are looking at to decorate I wouldn't get less than 7.5 Litres as it would be handy to have some left over for touching in.