What he said. The Vosges is considered a valuable environment and is strongly protected, local environmental groups are very active I doubt you'll find much in the way of legal trails. Penalties for ignoring access notices can be severe (including confiscation of the vehicle).
I used to be involved in an off-road group in Alsace and even a Sunday afternoon bimble in Land Rovers took weeks of paperwork and negotiation with farmers, mayors, hunters, walkers etc.