Understanding that 'samples' are a fixed recording in time and space ----- but having the option to vary vibrato would be key to our ever needful desire of 'expression'.
Right now I use solo strings (pfp patches) mixed carefully behind sections to give said expression (that really cannot be achieved even with xfade control - at least to my ears.) The vl-20 progressive legato patches are nice as well - but of course I want MORE control - suited to what ever music I am working on.
Kay's desire here is shared by all of us looking for greater expression. Xfade (with breath controller / pedal) on say - solo woodwinds is just not a workable option (phasing issues) - VSL I am sure is working continually to develop this ability.
Since I now have all the 'basics' of sample libraries - I look for developers who offer extended control over expression. When 'expression' is not offered in a new library release - I simply refuse to purchase (no value added to my template.)