Following the main storyline in Shadow of the Tomb Raider, you'll find yourself in the Mission of San Juan. There is one crypt in this location and you'll unlock it by completing a side mission - Against All Odds.
This crypt is located in the northern part of the location, in the middle of the jungle.
To get to the sarcophagus you have to descend on two climbing walls. On the first wall you can use your axe with a rope or descend with the ladder. At the next wall, however, you can jump down right away, as there is a deep water at the bottom.
After you get into the water, a sarcophagus shall appear. It contains Kantu's Gilded Vest.