I currently have a sql server 2000 sp4 instance with a read-only db on it. We get tlogs shipped from an outside vendor and keep a copy of the production database in house for reporting purposes. The database is in read-only mode because we apply new tlogs daily. Recently we did a full recover and applied 2 tlogs.
There is a user YYY in the database and at the server level, but they aren't linked. So I do what I usually do and run the following command:
Easiest would be to drop the login on your server, and re-create it with the SID of the user in the database. Alternatively, you can try to have the vendor drop the user and login on their end, and recreate it with your login's SID. The new SID will be brought to your server with the appropriate TLog restore.