On Windows VPS, the default comes with MS SQL Express Edition. If you purchase Plesk Power Pack addon, then you can get MS SQL server:
MS SQL Server™ support
With one of the world’s most advanced relational database management systems, Plesk 8.2 for Windows enables users to deploy and manage MS SQL Server databases right from the user interface with a few clicks. Plesk 8.2 for Windows can use and manage both local and remote instances of MS SQL Server Standard or Enterprise edition. Purchase MS SQL Server support to manage your local and remote database installations.