Any halal animal which is not slaughtered according to Islamic requirements is considered ميتة (dead meat).
الميتة في الشرع اسم للحيوان الميت غير المذكى، وقد يكون ميتة بأن يموت حتف أنفه من غير سبب لآدمي فيه، وقد يكون ميتة لسبب فعل آدمي إذا لم يكن فعله فيه على وجه الذكاة المبيحة له
Maytah in Shariah is the name of an animal which dies without slaughtering. It includes one which which dies without any act of a human, and it also includes that which dies because of the act of a human when this act is not the type of slaughtering which makes it halal
— Tafsir al-Jassas
Maytah is unclean based on 6:145, since it has been called رجس which is synonymous to نجس:
قل لا أجد في ما أوحي إلي محرما على طاعم يطعمه إلا أن يكون ميتة أو دما مسفوحا أو لحم خنزير فإنه رجس أو فسقا أهل لغير الله به
Say, "I do not find within that which was revealed to me [anything] forbidden to one who would eat it unless it be a dead animal or blood spilled out or the flesh of swine - for indeed, it is impure - or it be [that slaughtered in] disobedience, dedicated to other than Allah.
— Quran 6:145
And consensus has been cited by many scholars that Maytah is Najas (e.g. al-Razi, Nawawi, Ibn Hazm, Ibn Rushd ).
Also cooking meat and vegetables together causes parts of the meat to break down into juices and disperse into the vegetables, hence you will also be eating part of the meat whenever you eat the vegetables.
It is permissible to eat fried meat and fish from these restaurants,
subject to two conditions:
1.That it is not fried in the same oil as the non-halaal chicken or in vessels in which chicken was fried, until they have been washed,
because the non-halaal chicken is maytah (dead meat) and is najis
2.The meat should be halaal.