Don't know about the Vango but the MP2 is made with polyester .In theory that should not absorb water however having done some rain tests with that and other types of tent fabric it appears that it does.Those PU coated fabrics are coated on the inside so my oppinion is that they do wet out on the outside surface but the PU coating prevents them from dripping inside .After a few hours of heavy rain that material will be around 60% to twice as heavy as when dry , however it appears to dry faster than for example silnylon (your Akto...) although if you wipe silnylon down it then dryes very fast because water somehow clings to silnylon but does not penetrate because the fabric is impregneted with the silicone waterproofing so both sides are protected. Taffeta PU coated nylon behaves in a similar way to polyester.
"Silnylon" is not a single thing, it's a name given to lots of nylon with silicone impregnation/coating combinations. I don't know enough to know the relative merits of different approaches or how much difference different numbers of coating makes (the Akto will be double-coated on the outside and single-coated on the inside, IIRC, but I may not...). I do know they're not all created equal!
some also add a PU coating to their "silnylon" to increase the waterhead rating however that has a side effect explained in the first link.
I always thought the main point of a PU coating on silnylon was so you could subsequently add seam tape. Pretty brain-dead though, 'cause it heat-ages the fabric and makes it weaker. Better alternative is just to do the seams properly so they don't leak!
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