I'm looking into getting a few small VSL instruments to fill some gaps. (I currently own VSCO2 Pro.) Specifically, I'm looking at the classical guitar, jazz drums and saxaphones. I would spread the purchases out over a few months. What kind of benefit would VI Pro offer for the classical guitar or the jazz drums? (Will I see the most benefit to use it with the saxophones?)

Absolutely.  The humanize settings will help you create (and automate) varying amounts of detuning to note attacks.  The time stretch-feature will allow you to alter the length of the pre-recorded crescendo and decrescendos to fit your music.  The interval map feature will allow you to automate the process of adding slight detuning to short notes that is direction-sensitive (slide-up if the interval is up in pitch, and vice versa).

If you really want to get in-depth, for example, you can combine speed-detection, time-stretching, interval mapping, and humanizing all in one and create a matrix that allows you to play a short articulation and have the articulation subtly slide-up or slide-down depending on which direction you're playing AND play progressively shorter/truncated versions of the articulation depending on how fast you're playing.

It's a good tool :)

- Sam