Can I restrict users from reusing old passwords?

Yes, Vtiger's Password Reset Policy allows you to do this. 

Follow these steps to allow old passwords:

1. Log in to your CRM account.
2. Click the User Menu on the top right corner of the CRM screen.
3. Click Settings.
4. Look for the User Management section.
5. Select Authentication.
6. Click the Edit icon on the top right corner of the page.
7. Go to the Password Reset Policy section.
8. Enable the Allow Old Passwords checkbox to allow reusing old passwords, disable it otherwise.
9. Click Save.

Tip: It is recommended that you do not reuse old passwords as they may cause a security risk.