Can hard towels be made soft again.

I do use fabric conditioner and have a water softener but my fave towels are getting really hard. Weather permitting I dry them outside, otherwise in the boiler cupboard or over the banister. Not a great fan of using tumble dryer. I have heard of using white vinegar in the wash together with detergent but no conditioner. The towels in question must be between 5 and 10 years old. Anyone got any advice.