Whether "Krishna stealing clothes of gopis" is mentioned in Vyasa's "Mahabharat"? in which chapter?
If not, then which is the oldest purana in which this event is mentioned?
No, Lord Krishna stealing clothes of Gopis is not mentioned in Mahabharata. However, this event is mentioned in Srimada Bhagavatam, Canto 10 in details.
SB 10.22.7 — One day they came to the riverbank and, putting aside their clothing as they had done before, happily played in the water while singing the glories of Kṛṣṇa.
SB 10.22.8 — Lord Kṛṣṇa, the Supreme Personality of Godhead and master of all masters of mystic yoga, was aware of what the gopīs were doing, and thus He went there surrounded by His young companions to award the gopīs the perfection of their endeavor.
SB 10.22.9 — Taking the girls’ garments, He quickly climbed to the top of a kadamba tree. Then, as He laughed loudly and His companions also laughed, He addressed the girls jokingly.