Hi Tony- I have that combination and it seems to be working very well indeed. Now that I have gotten everything working well together (a monumental effort in its own right, as I remember, but not the fault of VSL/Kontakt 2/Mach5-these are just very complicated state of the art inventions and it takes some dedication to get it all together) I find the most difficult part is understanding the articulations of VSL and how they work together, which is what I am trying to do now.
The difficulty, by the way, of understanding the articulations is not the fault of VSL, who are very supportive, but the complexity of the enterprise itself. Using VSL is somewhat like being able to play all the instruments of the classical Europeon orchestra (with its centuries of refinement behind it). My method is to start out simple (maybe even a solo line) and try to make it sound interesting with a few of the many articulations available, and then go from there.
And of course it takes a little effort to make Mach5 work well too. My understanding, however, is that VSL will not work with Mach5. I got Mach5 thinking that it did and decided to keep it anyway because I like some of the sounds (especially the drums and percussion sounds, which are not in VSL because they are not included in the classical orchestra-the rest of the VSL sounds are superior, in my opinion, to whatever counterparts they might have in Mach5). Kontakt 2, on the other hand, works very well with VSL (and also comes with some very good drum and percussion sounds). Good luck! I don't think there's anything that I can see about the combination you describe that has been that much of a problem for me (except, of course, that Mach5 won't work with VSL, but let me know if that has changed). Best, OJ