It will take only few clicks to get your vCenter's table and relations in a Visio file. (I'm using visio 2007 I think the older one will work the same way as well) (Tested on visio 2007 SQL 2005 with vCenter 4.0)
Open your Visio 2007 --> File-->New --> Software and Database --> Database Model Diagram (Metric). This will open a new file for you.
Now click on Database --> reverse Engineer
Pick "Microsoft SQL Server" from Installed Visio drivers
Now click on "New --> System Data Source (Applies to this machine only) --> Next --> Pick "SQL Server" for the data source driver --> Next --> Finish
Now you will see the New data Source box
Give it a name, and the name/ip of your vCenter database (The normal way you setup your ODBC to your vCenter database). Make sure your change your database to your vCenter database.
Now you should see your ODBC connection that you just created --> click Next. It will ask for the connection password. Type in and click OK.
You might not want views or stored Procedures. so you can uncheck them.
Click Next and select all tables
Pick "Yes, add the shapes to the current page." few more clicks you should have your vCenter tables in your Visio file.
Open your Visio 2007 --> File-->New --> Software and Database --> Database Model Diagram (Metric). This will open a new file for you.
Now click on Database --> reverse Engineer
Pick "Microsoft SQL Server" from Installed Visio drivers
Now click on "New --> System Data Source (Applies to this machine only) --> Next --> Pick "SQL Server" for the data source driver --> Next --> Finish
Now you will see the New data Source box

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Now you should see your ODBC connection that you just created --> click Next. It will ask for the connection password. Type in and click OK.
You might not want views or stored Procedures. so you can uncheck them.

Click Next and select all tables

Pick "Yes, add the shapes to the current page." few more clicks you should have your vCenter tables in your Visio file.