The term priming/undercoat the walls means in effect if your walls have been plastererd, and you need to prime/seal the walls prior to putting on your normal coats of paint.
If your walls are just plastered and dried out properly, either get some white matt you have lying around and thin it down, and put it on the walls, there is different percentages as to how much thinning for this.
some people say 10% others 20%, I normally do 15- 20%, this seals your walls prior to your good coats, and shows up any imperfections caused by gouges, trowel marks etc, then you can fill them.
After rubbing down the filler, whack a bit of paint on the filled areas, when dry, start rolling your normal colour.
By using the white as an undercoat/primer, it saves you using the colour as the white is much cheaper, but its up to you, you can do a undercoat of the colour if you want, but obviously it will eat into your paint colour and cost you more money.
You can use a vinyl paint on your plastered walls, if they have been skimmed,but would be dubious about using a silk or soft sheen, not for a few good while anyway.