Hi, as you probably know GigaStudio works only on a PC, but the latest version of GVI (Giga Virtual Instrument) apparently runs on a Mac. I use GVI on a PC and it reads all my old Giga libraries (incl. the VSL Pro Edition) but lacks GigaStudio's editing facilities. You can even hook up the old VSL Performance Tool with GVI if you use a bit of freeware called Maple Virtual MIDI Cable. I also recall some people discussing Kontakt (not Kontakt Player) versions of the old VSL libraries here, but don't recall the details.
The Vienna Instrument (which requires the Vienna Key) is VSL's custom player. It doesn't play the old Giga and EXS24 sound libraries, you have to buy the new VI collections - I believe you get a discount if you bought earlier libraries. For legatos, repetitions etc. the VI is much easier to operate because the Performance Tool software is integrated with the player, you just load a patch and play.