While only Trillian Astra is specifically build for Windows Vista, Trillian 3.1 runs fine as well. However, you might experience a few issues.

Windows Vista might switch from the Aero Interface to the Basic Interface when you start Trillian. This is not caused by Trillian but by Quicktime (which is accessed by Trillian for some features). You can prevent this from happening by changing the Quicktime preferences. Go to the Advanced tab of the Quicktime preferences and set the Video mode to Safe Mode (GDI only).

When upgrading to Vista, Trillian might not save your chat logs anymore. This is caused by the User Account Control feature of Vista that prevents Trillian from writing to its installation directory. Instead, logfiles are now written to C:\Users\Your Username\AppData\Local\VirtualStore\Program Files\Trillian One way to overcome this issue is to create a Program Files folder in C:\Users\Your Username\ and install Trillian into that folder.