How HIGH is the Jump (fall!!)? A life jacket is meant to float, does NOT like going under water. Jumping in with one from a height (I don't know how high that is) would be like jumping / falling onto the ground / concrete / asphalt, etc. The vendor will (should) know what is safe vs. a sudden / shorter stopping distance.
It's NOT the fall that will kill you, its the sudden stop at the end!