i believe it to be a problem with the usb chipset and throughput of the vaio picturebooks (particularly c1vn and c1vpk in my experience).
i have no data to back this up as i don't know how i would run a test on it, but i had problems getting my usbpre to go 24/48 over usb on a c1vn and could not get a fw410 to do 24bit and certainly not reliable multitracking on a c1vpk.
the usbpre works perfectly on my full sized vaio at 24/48. i can't really comment on the fw410 as it was not mine and didn't have enough time to test it.
i believe due to the tiny form factor of the picturebooks as well as the relative novelty of usb at the time of their release to the marketplace, the bus throughput and chipset compatibility of the usb architecture isn't up to par with the devices being made today. i think that's what was causing your problem. i've tried and tried to get this to work but i can only spend so much time being a geek.
anyways, that said, there are plenty of success stories out there with people using pcmcia devices with their picturebooks and i think you are going the right route....vx pocket or any rme digi- or multi-set would also work well.
by the way, does the minime output 24bit over coax or is that only on the usb bus?