Brian,
It's fair to say that I am unhappy. Let's see if I can help you understand the position Autodesk has put me in. Vault is needed to get full value from certain AutoCAD verticals. I am disappointed to see that Vault has some intrinsic flaws (by design) that are a none-to-subtle impetus to sell yet more software (by your own admission).
---you said---
1) If you're not using Active Directory (Smaller company, Workgroup in a larger company, Novel, Unix, Linux... More?)
---response---
Vault either requires SQL or installs SQL Express. Both support AD, and if AD is not used, then the built-in security management for SQL is top-notch. Why complicate the matter with a custom model?
---you said---
2) Having a custom security model gives CAD Manager's the ability to fully administer the user model without worrying about asking IT for help (this can be difficult in larger organizations)
---response---
Since Vault operates on it's own instance of SQL, would it really be so hard to have the administration dialog handle the configuration of users via Sequel's own security? Then the question of whether or not AD is used is moot, it'll work perfectly either way.
Owen