Set up an adhoc network in Windows Vista to connect to and share the Internet connection with Mac OS X
Solution 1:
The following article can be used as a check-list:
How to: Create an Ad-Hoc Network, Part 2
If everything checks-out, it would help to know how OS X manages to connect to Vista but Vista cannot connect, what does "connect" mean, and whether you use static or dynamic IP (for dynamic, which is the DHCP server). Please also try ping and route and include their output in the post.