Hmm, well I'm not using the mono/C# build but under Editor Settings - text editor - external editor I see options for enabling external editor, field for choosing the external editor executable to use and a field for flags to use when launching the editor. But that's it.
Honestly though, you'll probably want to get used to the editor being always open anyways since otherwise every time you need to edit a script you will have to wait for the external editor to be loaded up again.
With it installed on an SSD and with it being the VSC which is fairly lightweight it isn't too bad perhaps, but it's still bound to get annoying after a while.
Or is the issue that every time you open a scene it switches the external editor to the front? I'd consider that a bug worth reporting to the tracker. In the meanwhile if you have a second monitor, maybe have the external editor in there.
