VSL guys (and Maya)[:)], your arguments are becoming pretty thin here... Are you saying that playing a single VSL patch, playing back 1-2 stereo voices at the same time, which causes random clicks when played for the first minutes, is more "complex" than hitting all of my white keys (more or less) on a chromatically sampled piano? So, playing back 30x44.100x2 samples per second, is less demanding than playing back 2x44.100x2 samples? C'mon...
I, like midphase am a LITTLE tired that you blame audio interfaces and harddrives, OS X etc., no matter if this problem occurs on a host of different hardware. I will give you this much, that I am currently using a firewire interface from m-audio. This might not be the most super pro interface and it does use FW (although you say that RME, firewire or not, would be fine), but doesn't it seem rather odd that all other sample playback engines work fine, despite my silly m-audio interface?
I will try this when I switch to a MOTU PCIe solution, and see if the problem persists. If it doesn't, OK, so your sample engine isn't working well with many soundcards, unlike Kontakt 2, EXS24, Synthogy etc etc. If it does, there is a problem with the Vienna playback engine across the board. Either way, something IS bad in the VI engine. I am not a programmer, so I don't have the faintest clue about what it is. Somehow, I suspect the "server" design concept to be the culprit here though. Your engine basically runs as some strange (??) daemon connected to an AU instrument in some way (feel free to explain the concept in detail, since I don't know much about it, I am just guessing from what I see in the Activity Monitor and how the "server" takes a long time to load the first instance).
You mention that other playback engines might use bigger buffer sizes. Well, if so, give us users the choice to increase the buffer size in the engine, if that is what does the trick.