Probably unsurprisingly, the press-and-hold-for-special-characters behavior is a macOS feature. It doesn't seem possible to change the behavior on the fly. You can adjust it per app like this:
defaults write com.sublimetext.4 ApplePressAndHoldEnabled -bool false
So the question becomes: how can you enter special characters in insert mode? In vanilla vim, you could use inoremap
to replace, say, Option+l with "ł". However, NeoVintageous (which I hope you're using, not the deprecated Vintage mode) does not support remaps in insert mode. It claims to support digraphs, which would be another way of achieving this. However, the keyboard command to insert a digraph is Ctrl+k, which conflicts with one of the default Sublime Text key bindings on macOS. (The one to kill from the cursor to EOL, which is copied from Emacs and available in standard macOS text fields.) It's presumably possible to work around this, but I felt like Ctrl+k+l+/ was kind of wordy anyway.
Fortunately, there IS a fast, simple solution: a Sublime key remap. Open your user key bindings (Sublime Text > Preferences > Key Bindings) and add this one:
{ "keys": ["option+l"], "command": "insert", "args": {"characters": "ł"} }
Now Option+l will add the l-slash character to your document. If you need the special character this is replacing (the not sign "¬"), you can of course choose another binding.
I presume this is the one you want, since it's the only special character available for me when I press-and-hold any of the hjkl keys. But this approach will work equally well for any other characters you might want to insert.