Seriously, to me it's alot related to the lack of warmth with the stringsin VSL which I personally find. My old sound modules weren't nearly as good but had more warmth. A real string section not only sounds real but sounds warm, and in my opinion the warmthness is as important as anything else, especially for sustained string sounds. I like to think it's because of the cold accoustic with the VSL instruments. I hope VSL is not ignoring this, because everything else about it IS so fantastic.
Guy
Just read this and had to come in here. This has been discussed many, many times regarding string sounds - there isn't a string library currently available that is able to do that to be fair. It's not really to do with accoustics in my view - it's more to do with string samples being almost tweaked to perfection - in other words, almost too perfect.
To get the sound you are referring to when simply playing a string patch is a wish everyone who writes using strings, has absolutely top of the sampling agenda probably.
How this will come about technologically I have no idea - currently layering is OK - but won't really cut it in terms of what you hear when you watch a performance of some symphony on TV.