Can you tell me how often I should use Yes^® water-based specifically for vaginal dryness, and how much to use each time?

Not really.  I don’t know how much will make it ok for you.  This is why we worked so hard to get our organic certification – you can use as much or as little as feels right for your body.  The purity of our products mean they are entirely suitable for daily use and can be applied as frequently as your body tells you it needs.
How much is used each time depends on whether you are using Yes water-based as a vaginal moisturiser (as much or as little as you need - typically 3-5mls per application) and/or for sexual intimacy. For making love you will need to use more as it will get "used up" more quickly.

Also if you are experiencing soreness when making love try and combine them by using Yes oil-based first and then Yes water-based on top, as this can give greater protection and increased longevity. Essentially it depends upon on how your body reacts and the degree of vaginal dryness you experience, as to how much you use and how often.