The itinerary lasts 17hrs 00min, and has 2 steps, and it will cost 80 euros minimum.

Nantes - Paris
The step from Nantes to Paris lasts 2hrs 07min. You'll travel in a high-speed train.

Paris - Vienna
The step from Paris to Vienna lasts 14hrs 53min. You'll travel in a night train from the company ÖBB.
On board, you'll have access to: Drinks and snacks.
You can find tickets from 80 euros.