When you are in Vastres take the direction of Fay sur lignon.
At the exit of Vastres about 300 m after, you will find a small road on your right with the sign "les Vastrets"
Cross the village and go around the penultimate house.
You will find a green gate and an interior courtyard. Behind the stable door you will access the key box. the code will be given to you before your arrival.