Having the manufacturer, make, and model of the fax server would help in trying to diagnose the problem you're having with Vista (we won't mistake it for SPAM). Can the Vista machines receive faxes OK but just not send them? What version(s) of Vista does it need to work with? Are you saying that the simple process of sending a fax from a Vista machine causes the Fax Server to completely crash or do you really just mean it doesn't work? What software are you using on Vista to send/receive the faxes (what comes with Vista or some 3rd party software or something that comes with the Fax Server)? Are you certain that you've networked the Vista machines properly to the server and that the server and fax software is compatible with the versions of Vista you are using?
I know almost nothing about Fax Servers (I've never actually used one or even set one up before and I've been doing this kind of for for 19+ years) and so can offer no real recommendations. I could research them just like you and come up with a suggestion based on what I found, but it would not be a well-informed or experienced recommendation which is what you're seeking. Why do you need a fax server anyway? Do you send/receive THAT many faxes that the programs available for Vista and W7 machines (and included in some of the versions) won't do the trick?
If you want to pursue this further, I need more information including answers to the questions asked above and anything else you can tell me about the setup, the configuration and size of the network (is it workgroup or domain?), any error messages that occur (the exact word-for-word text), how the fax server is connected to the network and to the computers and how Vista Network is configured to communicate with the server, and what ports the Server uses for communication (and whether Windows Firewall has those ports opened), and anything else you think might help. If you really aren't interested in fixing the problem but are really looking for another server, I'm afraid I can't help you there.
Thanks and good luck!