For the moment, the best driver you can find to use the U46MkII in ASIO mode under Windows Vista is the latest Asio4All 2.8 Beta
We tested and compared it to 1.0.26 MixVibes driver, given improvements in Windows Vista audio and Asio4All, that's the best choice you can do as long as our supplier is still working on the next generation of MixVibes U46MKII’s driver.
Even with 64 samples latency (~1.5ms @ 44100Hz), it works fine, without any crackling or failure (whereas the U46MKII's 1.0.26 driver becomes unstable even with higher latencies and even under Windows XP...).
We also tested minimizing MixVibes, switching between running applications, or turning on screen saver (what is obviously not recommended) while audio playing and Asio4All 2.8 's behavior is great (no failure, no crackling,...).
Asio4All 2.8 Beta is quite similar to the ESI U46DJ 1.0.16 driver (obviously based on a older Asio4All version) under Windows XP but much better under Windows Vista thanks to its compatibility with Microsoft audio improvements on this OS.
Conclusion :
under Vista >> U46MKII + Asio4All 2.8 = the BEST solution