The best solution is to put vCenter on its on virtual server.
also if you already have a license for SQL Server Std or Ent the use it and not the express edtion.
let the SQL Server do its job, on another server, and vCenter do its thing.
the exrpess edition has limitations to it, such as a 10GB database size.
i would not use another instance of SQL server unless the default instance has a lot of databases for one application.
i would just prefix my database with VMware_vCetner,...