On which day will you be making this trip?

This Friday

If you're currently in France, the best is to go to a ticket office for TGV reservations. That's also the cheapest option, since the Eurail website charges an extra 2 EUR per person per train. There are still seats left for 20 EUR on the train from Rennes (the Eurail website still has a problem to show these reservations).

For the night train Paris - Vienna, you can probably also book at the ticket office, but online it is possible via the ÖBB website, using this manual. There are still seats available for EUR 19.90; couchettes for 49.90 or sleepers from 79.00.

Vienna to Munich has optional reservations and can be booked via ÖBB website or ÖBB ticket office (3 EUR/seat). Munich to Paris to Rennes by TGV should be bookable via the French ticket office as well. Post your exact timetable (incl. date) for more detailed advice.

This is my whole itinerary, unfortunately I cannot book online.

I´m afraid I will have to go in person

You only need a reservation for the TGV Rennes to Paris and the Nightjet Paris to Vienna. 

And a reservation for the ICE Stuttgart to Paris and TGV Paris to Rennes. For all other trains you can get on the train without a reservation but you can buy one the be shure to get a seat (it should be not so full due to the current situation). The ICE to Paris has only a small number of reservations for Eurail users, buy it asap. 

Are you currently in France? Then go to the next train station and buy at a ticket office or automatic ticket machine of SNCF. Please buy the reservation also for your trip back to Rennes. 

You can also buy it online via Eurail Reservation Service, SNCB/NMBS International (via Railplanner App AMore>Seat Reservation>France] and for the Nightjet also via tickets.oebb.at (manual linked by rvdborgt)