However if the user record is not found or the user already has vip_until >= CURRENT_TIMESTAMP (i.e. the user has already purchased "vip status") I would like to cancel the INSERT.
You can "join" two DDL commands by using a Common Table Expression (CTE) (i.e., WITH / SELECT). You would need to make it so the UPDATE happens first and if there are no results the INSERT simply becomes a no-op.