I found the solution. I'm posting it here in case other people run into the same problem.
You need to turn the VirtualBox setting to one monitor (and not full screen). Then, watch Linux Mint to see which monitor it opens first. THAT is the monitor that will have the menu button. Simply move the single Linux Mint window to the correct monitor where you want the menu to appear. In my case, I want the menu on my left monitor. Then, shut down Linux Mint. Change the VirtualBox setting to two or more monitors, depending on your configuration. Then, start Linux Mint again. The menu button will be on the correct monitor!