From my experience, these buses leave on a per need basis. Once you are bussed to the terminal, you will see the transfer bus point to your right (before climbing the stairs, next to the information counter). Sometimes you have to go to the information counter to let them know that you need to be bussed.

However, you only need to be bussed if your non-Schengen flight leaves from the C terminal. Given that this a Tyrolean flight I would say that it will most likely depart from the D terminal, meaning no need to get bussed. Look at the Vienna airport website (e.g. tomorrow's flight VIE-LED leaves from bus gate D65) for more information.

What I would check is whether Montenegro Airlines is able to issue you the boarding pass for the VIE-LED flight. I flew OS once from TGD connecting in VIE, and Austrian was not able to issue me both boarding passes even though all flights were OS operated.

If Montenegro are able to issue you both boarding passes, you should probably be fine.