Mithun returned with brilliant figures of 16-6-45-5, his 11th five-wicket haul, and was finely aided by skipper R Vinay Kumar (2/36) as Karnataka bundled out Vidarbha under 200 in 61.4 overs.
But the eight-time Ranji champions were tottering at 36 for three in 14 overs when bad light stopped play.
Karnataka still trail Vidarbha by 149 runs with four days play still remaining.
Young Rajneesh Gurbani (2/9) put up a fine display of pace bowling under the shadow of Umesh Yadav (1/22) as 13 wickets fell on the eventful opening day on a green Eden top.
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At close vice-captain Karun Nair was batting on six alongside wicketkeeper batsman CM Gautam (9).
Having caught up by a traffic jam due to Kolkata 25K run event that caused a 30-minute delay in start of the match, Vidarbha surprised all and sundry when skipper Faiz Fazal opted to bat on a green top.
Vidarbha were going steady on 49 for two after some early morning jitters when Mithun opened the gates in the third over after lunch striking with his first and sixth balls.
Apoorv Wankhade (1) followed suit after an inside edge off Mithun while S Aravind took the prized scalp of veteran Wasim Jaffer (39) as he took an outside edge playing forward to be caught by R Samarth at second slip.
Mithun was on fire in his third spell of 4-0-12-3 as he took three scalps in a single over to reduce Vidarbha to 171 for nine.
Aditya Sarvate and Akshay Wakhare were going steady before Mithun's burst ensured a collapse.
Vidarbha's top scorer Aditya Sarvate (47) was caught in the cover with spinner Shreyas Gopal taking the last wicket.
Vidarbha were bolstered by the return of Umesh Yadav but Fazal's decision denied the India pacer a chance to make full use of the conditions on offer.
Karnataka skipper Vinay Kumar drew the first blood, trapping his Vidarbha counterpart and leading rungetter Fazal for 12.
Stuart Binny then removed another in-form batsman Sanjay Ramaswamy for 22.
It was left to their experienced duo of Jaffer and Satish as they steadied the ship to see through the first session with some element of luck.
Satish got a reprieve on 14, while Jaffer got an edge on 30 but it fell short of Mayank Agarwal at third slip.
Brief Scores:
Vidarbha: 185 all out in 61.4 overs (Aditya Sarwate 47; Abhimanyu Mithun 5/45) vs Karnataka: 36/3 in 14 overs.