Could be a replacement fork, but the original looking Vitus decal seems to match things on the frame (that also has a Vitus tubing decal on the seat tube) like the Vitus rear dropouts....
But still kinda weird seeing a Spanish fork crown on a very French bike... I was expecting to see the usual stylized "M" with the crown and the tiny circles on it. Unless there was a Mercier/Zeus connection at one time