I'm using version 8.1.0.421 right now. However, one of my contacts downloaded beta 9.0 at one point and we were both on Vista and still able to talk one on one (although 9.0 did have some odd issues and he downgraded to 8.1 again.)
You can grab a clean copy of YH at
Filehippo or on the
Yahoo Messenger website.
If you're both on Vista, please make sure you've allowed YH completely through the Windows Firewall or if you're using another firewall either hardware or software you need to be sure the Windows Firewall has been disabled so they don't conflict.
I'd also recommend trying to launch YH by right clicking the icon and choosing "Run as Administrator" and seeing if that helps at all. If you have User Account Control (UAC) running on one or both of the machines it can make things terribly difficult to get things to run properly. Unless you really need it for some reason, I'd disable UAC completely.
I still find 8.1.0.421 is far more stable on Vista and would suggest trying it instead of the beta 9.0 for Vista. Let me know if any of this helps you out. If not, can you give a little more detail as to exactly what does or doesn't happen when you try to call? (ex: does it ring but you can't answer, does it shutdown YH, etc.) it would be helpful. Good luck and we'll wait to hear from you.