To make the sustains longer (stretching) is possible with VI Pro, but most of the time not so good idea because the vibrato becomes slower.
The good news about this is, that with the same tool (stretching the sample) you can vary the vibrato, and that gives often a "reality push". For example starting with slow vibrato and then increasing the speed of the vibrato. Or slow down the vibrato a bit at the end of a tone. It's definitly an expression tool that sounds very well.
Another good working tool is to diminish the attack a little (not to much). But that is something different again.