Everytime the a user access the report, Reporting Service exposes a global variable User!UserID which is the same as the DotNetNuke login user account (thanks to ReportXP custom security extention).  Inside the sample Vendor report, the variable User!UserID was passed into the query to filter the resulting dataset.  Login using account 610 and 678 and notice the Vendor accounts return from the report for each login user.