When upgrading VirtualCenter to 2.5 Update 1 and using Oracle Express 10g for your VirtualCenter database, be aware that you will see an informational dialog box suggesting Oracle be upgraded to version 10.2.0.3 for proper VC functionality.
However, as of this post, the latest version of Oracle Express 10g is 10.2.0.1, Release 2. I have seen that the VirtualCenter upgrade will complete successfully, but you may experience some issues afterward. More specifically, I have seen:
a. The VirtualCenter will not launch as the VC service is not running. To correct, the Oracle XE service must be restarted, then the VC service will start and you are able to login to VC.
b. After logging in, the Update Manager plugin fails to load stating that UM cannot contact the database or the connection is in the process of closing.
c. If any additional administrative functions are performed, such as a vmotion, nothing is displayed under Recent Tasks.
To correct these issues, you can use either the Oracle Express Administration web page or the Oracle command line tools to increase the Sessions, and Processes parameters. The default settings are 40 Sessions and 49 Processes. On a recent install, these parameters were set to 100 Sessions and 110 Processes. After setting these variables and restarting the Oracle XE services, VirtualCenter worked without any of the symptoms described above.
The VMware VirtualCenter and Update Manager Database Compatibility Chart: