Da is in Darth already
ay and V are in Vader (you could probably get most of the name by playing Vader backwards).
Much of the time he is just called Vader or My Lord.
If you counted the times he is refered to as Darth we might be surprised and when he is it's usually by characters who have said it already or will again.
Ben says Darth more times than anyone (I think) and you can use the same construction everytime.