Hi,
USB is much more complex than a serial port. The main difference is that USB supports multiple devices communicating through he same lines, so the communication need to be arbitrated and packet-based. CVI does not offer any USB libraries, the main reason behind this is that every USB device has a different way to communicate; USB defined the bus transfers, but not any standard protocol for devices.
Some companies offer USB-to-RS232 converters that come with a driver that allows you to use RS232 libraries to perform serial communication.
As far as calling VC++ code from TestStand, it is definitely possible, I don�t know any of the details involved; but you may want to consider posting more details in the TestStand forums.
I hope this helps,
Regards,
Juan Carlos
N.I.