There are many aspects to this answer. If you are from South India then go for NIT Calicut. It has a reputation for good placement records. Here are the placement statistics for both NIT Calicut and VNIT Nagpur.

If you are not from South India, then choose according to your convenience. Don’t choose Calicut over Nagpur as it is not an easy place to live in for four years. If you like a lot of alone time then Calicut will be suitable for you. Nagpur is in central India. So, you will have easy connectivity to your home town. VNIT is located in the middle of the city. You will enjoy your stay there. 

Both are good institutes. Choose according to the proximity.