Thanks, @JohnAtl.
I bought Automounter and the “pro upgrade” for a total of $13.99. So far, so good.
When I connect to the VPN, it mounts the share with the correct user and remounts when I disconnect/re-connect.
I was reluctant to post such an edge-question here, but I can’t tell you how much googling and reading I did without uncovering this solution. Thank you!