Firstly......your plasterer is wrong, a thinned coat of emulsion is used to prime the walls not seal them?....seal them from what??...it soaks into the surface as opposed to sitting on the top...if a coat of paint is applied unthinned then the water in the paint soaks into the wall....leaving the pigment on the surface, which could eventually flake off.
Also, if you put a coat of unthinned paint onto bare plaster you will end up with a bad 'orange peel' effect....and on top of that.....its hard work too!
If the walls have had many layers of plaster on them forming a thick coating then you should use non vinyl matt emulsion for all the coats to allow he wall to 'breathe'
If its just a skim over then supermatt or vinyl matt will do..
NOT VINYL SILK!....but make sure you give the walls tree coats....one mist....two full coats..if you put a coat of vinyl matt over non vinyl matt then you will probably get a patchy finish.
Dulux supermatt is fine for a mist coat.....i use it all the time and you can go 50/50 easily....it should always be cheaper than vinyl matt..I pay about 9 quid for 5 litres of supermatt and 16 quid for vinyl matt.
Once the walls are 'misted up' then its ok to give them a light sanding down.
He is probably comparing a dulux retail paint (vinyl matt) to a trade paint (supermatt)....I have noticed that places like b&q keep trade paint in stock......but its an awful price in these places...dont touch it...look for your local dulux trade centre...trade paint is far better quality
In the trade we dont use the term 'non vinyl matt'...its called 'contract matt'.....if you ask for this in a trade cetre they will point you in the right direction......if you ask for it in a diy store they will probably say 'wha?'
And.....beware of advice from staff in these places most of them are clueless...not their fault....its the lack of training given.
And as for the 'painter'....if he is a painter.....why isnt he out there earning a fortune bleeding old ladies of their life savings?......like me!
Good luck...