I'm coming round to the idea that you need VelXF for a typical tring quartet, otherwise the sound is simply too bland -- the biggest probem being a real muddiness in faster passages. On the other hand, setting the VelX slider on maximum gives a real brittleness to the violins in particular so I've decided on around 90 for the violins and nearer 100 for viola and cello which is the best compromise I've found to date and guarantees that slow sections at least are genuinely expressive.
Aside from keyboard and plucked instruments, you need VelXF for just about everything including strings. You also should use it in conjuction with the Exp fader as well to really bring out the detailed nuances of the instrument. Also you should be making subtle adjustments to Attack while the piece is playing in real time. If it's too much it tends to cause the muddiness in faster passages that you might be hearing. Too little makes things choppy. The release fader I set to about 70 and leave it alone.
But the real kicker for solo strings, or any solo instrument for that matter, is tunning. Ever so slight manipulation of the tunning fader (15 to 30 cents) can pump life into your MIDIstrations. In addition to increases and decreases in tempo at appropriate parts of the piece; slowing at the end of a melody line that usually falls on the root note for example.
If you have VIPro you can achieve a lot with the humanization function and time offset. But all of these tools are useless if you don't know how to use them and learning the craft of MIDIstration takes blood sweat and tears I'm afraid. It's just like learning an instrument.