The syntax for VSwitch is the following (see the Books Online Help for the full syntax of this utility):

vswitch -S {60 | 65 | 80} 

You can use the VSwitch utility to switch between the currently active version of SQL Server and another version of SQL Server. This is important when SQL Server 2000 has been installed on a machine with a 6.0 or 6.5 version of SQL Server because the earlier versions of SQL Server (6.0/6.5) cannot run concurrently with SQL Server 2000. SQL Server 7.0 servers can run concurrently with SQL Server 2000 on the same machine, but they are also unable to run at the same time with 6.0 or 7.0.

It is recommended that only one version of SQL Server be installed on a given machine. Each version uses a different set of Registry keys, so configuration changes made under one version will not be made under the other version. This utility is provided primarily as a means to assist in the upgrade process.

The VSWITCH.EXE file is located, by default, in a path that is specific to the SQL Server instance. Therefore, one EXE is installed for each instance that is created. The path will typically be Program Files\Microsoft SQL Server\MSSQL$instance_name\Binn\.

    Part III: SQL Server Administration
    Part IV: Transact-SQL
    Part V: SQL Server Internals and Performance Tuning
    Part VI: Additional SQL Server Features