And if the softener is sized and programed correctly, you never have to do this. I find that most softeners are not sized and programed correctly but, if any softener has ever been allowed to run out of salt, you will have hardness scale build up in a water heater of any type.
If you have more than 3 gpg of hardness in your water, you need a softener for more than just the water heater's benefit anyway.
Just running vinegar through the heater isn't going to remove much hardness scale. It has to be allowed to 'soak', then flushed and replaced and more soaking. How you tell when enough is enough is beyond me.