Sith on the other hand don't think like Jedi, but I never read up on a Sith vs a Mandalorian. If their armor could withstand lightsabers, all they have to do is aim for places that don't have armor,as u mentioned, the neck, side, and perhaps exposed parts on arms and legs and that requires GREAT skills with a lightsaber cuz I doubt a Mandalorian is gonna stand still. It would be an interesting fight though.
"At last we will reveal ourselves to the Jedi. At last we will have revenge." - Darth Maul