The easiest way to simulate the UAC is to install an MSI from an elevated command line. In Vista choose to run the command prompt as Administrator. Then install an MSI. You can flag an MSI, by modifying the WordCount in the Summary Information Stream, so that it does not automatically require elevation during an install. This MSI would only be able to write to the user profile and not any protected system location. In corporate deployments this is a rare scenario.
The application installer will always need to be elevated but this will be handled by the deployment tool. If an application does turn out to be badly written then this scenario can be handled in the same way as under a locked down XP machine. Specific NTFS permissions would be given for the file, folder or registry key.