Try looking on the support page, for the products you are interested in.
I got lucky on the first try. The magic word Vista is mentioned here.
http://www.usr.com/support/product-template.asp?prod=5610b
The driver is a tiny download, and looks to be mainly an INF file.
This is the header of the file. The rest of the file appears
to modify the registry, so it isn't much of a driver in that
sense. (And that is because this modem is a hardware modem.)
*******
; USR5610.INF, version 6.22.70.03
; Copyright 2000-2006, U.S. Robotics Corporation
; Windows 95,98,2000,XP(32&64),Vista(32&64) INF Support for U.S. Robotics
V92 Fax PCI (5610B).
; Modified by U.S. Robotics Corporation PQA-CF on 8/17/2001 1:55PM.
; Modified by U.S. Robotics Corporation -TS on 3/25/2003 12:03 PM
; Updated DriverVer 5.22.66.00 by U.S. Robotics Corporation -TS on
8/06/2003 2:47 PM
; Updated DriverVer 5.22.68.00 by U.S. Robotics Corporation -TS on
8/07/2003 4:48 PM
; Updated DriverVer 5.22.70.00 by U.S. Robotics Corporation -TS on
11/20/2003 5:11 PM
; Added support for XP x64 OS 5.22.70.01 modified by USR-TS on 08/04/2005
3:25 PM
; Fixed Modem response NO DIALTONE and DIAG by MS PRojects -TS on
10/04/2005 1:16 PM
; Changed .NTx86.5.1 to NT.5.1,NT.5.2 by USR-TS on 05/09/2006 11:20 AM.
; 11/15/2006, 6.22.70.03 added support Vista&fixed SourceDiskName USR-TS
*******
There are some Winmodem type designs, where the driver is
actual code (executable), and it decodes the tones on the phone line.
That is bound to be more complicated, and since that is a real driver,
it might take more work to make a Vista version.
For example, I tried looking for driver support for the USR 5699b,
which is stated to be a Winmodem. And they have a Vista driver for it!
The driver in this case, is 0.9MB in size, so the driver is a lot
larger than the tiny INF file used by the other one. (One thing that
is weird in the following links, is the driver is stored in a "5699c"
folder, even though the product is a 5699b.)
(Download page)
http://www.usr.com/support/product-template.asp?prod=5699b
(Download file)
http://www.usr.com/support/5699c/5699c-files/5699-Vista-x86-v338.zip
(Product page)
http://www.usr.com/products/home/home-product.asp?type=specs&sku=USR5699B
The reviews for that Winmodem don't look that good. It is cheaper, to be
sure,
but not many people liked it.
http://www.newegg.com/Product/ProductReview.aspx?Item=N82E16825104137
Some reviews for the 5610B are here. The design appears to be a few years
old.
Since the hard part is done in hardware, on the card, it costs more, but
should be less bothered by driver issues.
http://www.amazon.com/Robotics-PERFORMANCE-V-92-56K-MODEM-USR325610B/dp/customer-reviews/B00006HU75
Actually, in some tests I did a couple years back, I got 1% more thruput
on a Winmodem, versus an external USR Courier modem. Which surprised me.
So a Winmodem can in fact, be very good. The hard part, is figuring out
which Winmodems have decent software. You can spend $10 for the Winmodem
many times over, trying to find one with good driver code. But they do
exist.