Personally, I have found USB NICs to be particularly slow at data transfer and would never consider them for hard disk imaging. Have you tried loading a different driver - say for an INTEL or BROADCOM NIC, just to check whether it might be a more generic issue? However, it seems odd that you are being told 'Failed to find any drivers to load'.

I suspect that simple block storage USB devices such as flash memory sticks and hard disks can be mounted without needing additional driver support, but trying the more complex task of layering a network card driver on top of a USB driver may be a step too far. Since WinPE is intended as a pre-installation platform and not a full blown operating system, its support for plug and play is quite limited.

I had a look at the INF file associated with the Vista driver, and it only references one SYS file in the INF, so I'm guessing that the problem is not related to missing files.