/ >/What purpose does vpopmail serve that dovecot alone does not? Isn't it

just an ancient leftover from historic qmail times?

qmail and vpopmail are still quite prominent on the Internet. I see no reason to change my setup at the moment. Possibly I could re-architect to read user info directly from mysql but that would not help with the fact that dovecot then likely read or write as the wrong userid on the system (since mailboxes are virtual).

Have recreated the same issue (sans mysql) on another vpopmail system.

Regardless, it is a bug.

From my process list:

25813 vpopmail 20 0 26468 4984 4016 R 0.7 0.0 0:00.02 imap 26730 vpopmail 20 0 28332 5940 4212 S 0.7 0.0 0:00.06 imap

For certain, imap being ANY other user than root means that the /var/run/dovecot/imap-* sockets would be unwritable.