De acordo com o homem:
Flags give you a way to modify the behavior of the spawned process. They are used in the commands ":open_with" (key "r") and ":shell" (key "!").
f Fork the process. (Run in background) c Run the current file only, instead of the selection r Run application with root privilege (requires sudo) t Run application in a new terminal window
No seu caso, você precisaria usar :open_with r vim