Hi Nigel.
A really thorny issue this VAS and could be the subject of a PhD. thesis!. Not to mention a few headaches!
I always follow the NickN bible to set things up, have stuck to DX9 and have so far never had many OOM's. But I have an old FSX setup, that I have weeded over the years, so the sim is not clogged with rubbish.
The things I see that use most VAS, which is of course limited to 4Gb in FSX, (as a 32 bit program= 4 Gb Max) are airport scenery, advanced aircraft sims, eg. PMDG/Glass cockpit sims, and the scenery used along with the scenery sliders chosen. However an