I have listened Vikarna was the only Kaurava who questioned the humiliation of Draupadi, the wife of his cousin Pandavas after they lost her in a game of dice to Duryodhana.Is this the reason why Bhima did not hit Vikarna
Yes, Bhima has slain Vikarna also. It is because he took the vow to kill all the Kauravas. He had some compassion for him because he was the only Kaurava who talked against the draupadi disrobing incident. Vikarna was dear to Bhima because he has helped and friendly to the Pandavas.
The seven sons of thine that were thus slain were Satrunjaya, and Satrusaha, and Chitra, and Chitrayudha, and Dridha, and Chitrasena and Vikarna. Amongst all thy sons thus slain, Vrikodara, the son of Pandu, grieved bitterly from sorrow for Vikarna who was dear to him. And Bhima said,
Even thus was the vow made by me, viz., that all of you should be slain by me in battle. It is for that, O Vikarna, that thou hast been slain. My vow hath been accomplished. O hero, thou camest to battle, bearing in mind the duties of a Kshatriya. Thou wert ever engaged in our good, and especially in that of the king (our eldest brother). It is scarcely proper, therefore, for me to grieve for thy illustrious self.'
Yes, Bhima did slay all the Kauravas including Vikarna, the third oldest kaurava and the only kaurava who protested against draupadi's cheerharan and was actually sad.
Vikarna's death was the only death Bhima regretted killing.
Bhima even performed the final rites of vikarna.