Chicken with Sherry Vinegar and Tarragon

Hello again, Thank you for you earlier answer re the Tarragon Chicken recipe. I am still worried about the quantities of sherry vinegar - how much might I reduce the amount and obtain a less astringent flavour? My Cordon bleu friend has just made it for us to try, and has experienced the same problem - fiercely vinegary, despite the most gentle reduction, and care.