Place the chicken in a resealable bag and pour the vinaigrette over it to coat. Press out the air and seal. Marinade the chicken for 1 hour or up to 24 hours in the refrigerator. (Chicken can be frozen at this point. Thaw completely before moving to step 2)

Grill the chicken over a medium flame for 4-5 minutes. Flip the chicken over and continue cooking until cooked through, approximately an additional 6-7 minutes.

Transfer the chicken to a platter and allow to rest for 5 minutes. Slice thinly across the grain.

Spread 1/4 of the tzatziki sauce on one side of the pita. Repeat with remaining pita bread.

Arrange the lettuce, chicken, onion, tomato and cucumber on the pita. Sprinkle with the feta cheese.

Fold the pita around the filling and serve immediately.