Do you have 32 bit or 64 bit vista? I have 64 bit XP and have always had frequent issues with sound cutting in and out when playing online. That is with the on board sound, SB 24 bit, and USB sound card. Obviously, 64 bit XP sound drivers have issues.
I have bad results if I try to use onboard sound or do not have it disabled in bios. I have my best results when I disable onboard sound in bios and only use either the SB card or USB, If the SB card is in the slot and I try to use the USB, I almost always have sound problems, intermittent sound and no sound. A reboot usually solves it, but not always, sometimes it returns. The USB sound card is not really plug and play with XP_64.