Add the chicken breasts, chicken thighs, water, chicken stock and vinegar to the Instant Pot or EPC insert.
Set to Sealing.
Manual, High for 15 minutes.
(If cooking from partially frozen or frozen, cook on Manual, High for 22-25 minutes.)
Shred with 2 forks, strain off excess liquid. Season with salt and pepper to taste.
If you plan to freeze it, let it cool completely before adding to baggies with bag holders and freezing.