Windshield Washer Problem

The right windshield washer on my 2012 Prius has stopped working 4 times in the two years I've owned this car. The three way valve has been replaced by the local dealer under warranty each time. Every time they ask what I use in the reservoir, and I tell them 3 parts water and 1 part white distilled vinegar. They have told me each time that was fine, until today. The service tech told me today that they will no longer replace the valve under warranty, because of the vinegar. He said I'm the only one with this problem. Also, at 39K miles, the rear washer stopped working, and they told me I need a new pump, and refused to replace it under warranty. Has anyone else had a similar issue? Thank you for your feedback!