If you have users that access SSRS reports using only the SSRS Report Manager, you will need to verify that these users can still log into the Report Manager and access the reports.

For example, if these users have been given folder level security access to reports, they will not be able to access those reports once Vista changes security on those reports to item level security. If these users cannot access the reports, use the SSRS Report Manager to grant those users item level security on the reports.