The simplest way to do this is to uninstall, Fortress, move the databases to the new SQL Server machine and reinstall Fortress. The sgvaultuser password is the same as the Fortress Admin password, so you can change it at that time.
Or, you can move the databases then edit the connection string for "Server" in the Fortress web.config file in the Vault Service directory on the Fortress Server machine.
<SQLSERVERMODE-add key="ConnectString" value="Application Name='SourceGear Vault Server'; Connection Reset='true'; Server=(local); Database=sgvault; User ID=UUUUUUUU; pwd=PPPPPPPP" /> SQLSERVERMODE>
More info here:
http://support.sourcegear.com/viewtopic.php?t=924
If you wish to change the sgvaultuser password without reinstalling, there's a workaround: change "pwd=" to "password=" in the connect string and you can enter the password in the clear. If you want the password to be encrypted, then uninstalling/reinstalling is your best bet.