This page will have a guide to the treasure map you find in Ile a Vache in Assassin's Creed IV Black Flag
Treasure Map Location[edit]
The treasure map itself can be found in a Cadaver on the Northeast corner of the island, at coordinates (843,140):
Treasure Map Tool[edit]
This is the treasure map you find:
Coordinates indicate (565,539).
Note that the treasure chest itself is NOT located in Ile a Vache (where you got the treasure map from), but rather in Jiguey:
Dig Site Location[edit]
Dig location is around coordinates (565,535):
Near a formation of palm trees, also there is a campfire nearby. See screenshots below to help you find where to dig for the buried treasure:
You get 4000 Spanish Reales