http://www.usr.com/support/product-template.asp?prod=oem yields only OTHER OSes are supported.

It also reveals that Gateway or those other makers are your source of support and not USR.

Charts!
http://www.usr.com/products/business/compare-business.asp
http://www.usr.com/products/home/compare-home.asp

Aha! Not a single Vista item in sight.

-> http://www.google.com/search?hl=en&q=vista+voice+modem&btnG=Google+Search finds the Hiro. Maybe others.

Sorry but as no one does dialup here I can't offer much else.

Bob