I wouldn't rule out Vosges.
Mmmm.... doubt it, even more so if there will be 2 stages in Cantal. In that case we would have:
Stages 1-3 in Italy
Stage 4 light Alpine stage
Stages 5-7 towards Burgundy, stage 5 may be another Alpine stage
Stages 8-9 in Burgundy
Presumably some stages going south or Southwest
Middle of the 2nd week in Cantal
Maybe 1 transitional stage
3rd weekend in the Pyrenees
1 or 2 transitional stages towards the Alps
2 Alpine stages
If we had 1 transitional stage to the Alps, we will have another before the final weekend, if not those 2 Alpine stages will be followed by the already known stages 20 and 21.