Welcome to dba.stackexchange.com. What you are looking for is the SQL Server SA password which was probably installed with VCenter, or it was installed prior and VCenter was configured to use that SQL Server.
If you have admin rights to the server, you can try logging in 'as a windows user' and seeing if the admin was configured to have SA rights. If you can get in, you can change the SA password.
If you do not have access to SQL via Windows Logins, you can still use what some people consider an exploit, and access SQL Server via the NT Authority System userCheck out some blogs on it here, it's pretty easy to do. This is the command as copied out of the blog:
PsExec -s -i "C:\Program Files (x86)\Microsoft SQL Server\110\Tools\Binn\ManagementStudio\Ssms.exe"
Recover access to a SQL Server instance. Written by our very own Aaron Bertrand.
Option 3 (The nuclear option):
If you are sure there is nothing you need on this SQL Server and it was only installed for VCenter you could always uninstall and reinstall. This is a last resort however, I don't see any reason why you would have to entertain this idea for your current issue.