In small bowl, whisk soy sauce, half (1.5 Tbsp) rice vinegar, hoisin sauce and cornstarch until smooth
In a second small bowl, mix tomatoes, cucumber, sugar, and remaining (1.5 Tbsp) rice vinegar
Heat oil in large non-stick pan or wok over medium-high temp
Add scallions, ginger, and garlic to pan, stirring constantly for 30 sec.
Add chicken, keep stirring until cooked (approx. 3-4 minutes)
Add soy sauce mixture & water chestnuts to chicken in pan
Cook & stir for about 1 minute until mixture thickens & transfer to bowl