Vista shared printer not accessible by other Vista PC

I have a small 3 PC network with 2 Vista Business machines and 1 XP Pro
machine. XP can access the shared folders and printer on both Vista machines.
Both Vista machines can access the shared folders on the XP machine. However
neither Vista machine can access the other's shared folders or printer
without being prompted for a username and password.

Both Vista machines are configured for the same workgroup as a private
network with file sharing, public sharing and printer sharing turned on.
Password protection is turned off. The firewall settings show printer and
file sharing access enabled.

So why does this happen? If XP can work with Vista and Vista can work with
XP why isn't Vista working with Vista?

(FWIW: For any MS people, networking was just fine in Windows 2000, since
then it's gone downhill, imo)