Can I size new plaster with 50/50 water and PVC glue?

Please do not even think about sealing with PVA! (I assume that's what you mean by PVC?). This is incorrect practice, no matter which way you slice it, and will store you up a raft of future problems ('crackling' being the least of your problems) which will be costly & time-consuming, not to mention demoralising for you, to have to put right! If you're papering (either lining or otherwise) then bare/newly plastered surfaces should be sized using an appropriate sizing solution - or, if pushed, a dilution of wallpaper paste & water is a serviceable alternative. PVA is not. If painting new plasterwork, you must mist-coat (40:60 solution paint to water) at least once before following with 2 full coats. It is an industry standard to use basic (ie: cheap) white contract matt for your mist-coats - although it is also acceptable to use a thinned solution of your finishing colour. Much like PVA - and for similar reasons, do not ever use silk/eggshell or any other mid/high-sheen paint for your mist coats. Best of luck with your project & if you have any further questions feel free to contact me. Kind regards, Alix St.Claire Alix James Decorators

Do not use any p.v.a what so ever you will lose all suction for the paint to adhere to the plaster and in time the paint will peel off, Just water down some contract Matt emulsion containing no vinyl and use this as a priming / mist coat,then apply three coats of a good quality vinyl Matt emulsion diluted slightly

I can't see that being a problem. Best way is to try it on a small wall & paint then leave it overnight to dry.