Keeping your DB on MS SQL Express within the same SSO VM or on an external DB is a matter of preference and your specific configuraiton. Things to consider to help you make the decision:
- Do you have a production DB that is well maintained by your DBAs? If so then putting your DB there might make super sense and save you the hassle of maintaining it or backing it up as all well be handled by the DBA handling that box. Read why you might do other wise below though before you go with this option.
- Do you rather to have your SSO VM self contained in case if you are using Image backup/restore? In this case having the DB on the same VM might be a good option.
- Are you worried that the maintenance of other APPs DBs or a centralized DB Server maintenance affecting your SSO availability? If so then having the SSO DB residing on the SSO VM would be your best bet.
I have covered this best practice and much more on my blog post at: vCenter Single Sign On best practices
Regards,
Eiad Al-Aqqad