Cover delay relates to number of rings at that extension before forwarding to another extension (not vmail), and is not used on the vmail ports themselves. If these values were altered for the vmail ports themselves, leave those values as-is, they'll have no effect on operation and not be worth going back to reset.
Be sure Auto VMS Coverage is set for the extensions using #310. Then verify VMS Cover Rings is set under #117 to whatever you need.
The reported failure to immediately forward to vmail when DND is enabled is troubling, though. Verify 78 & 79 are the only members included in Hunt Group 7 under #505. If an errant port is included, it'll ring that until forwarding occurs to the next hunt port, causing at least 3 if not 6-7 rings (if more than 1 port accidently included in the group) before vmail answers.
My suggestion is to verify programming using the remote software so you can see what may be out of place graphically. Great for allow/disallow tables, restriction errors and troubleshooting strange programming errors like this one.