We do not have enough data on this route to provide a reliable graph showing how many days before departure it is the best to book.
However, usually the closer it is to the departure, the more expensive the tickets are.
The most popular airlines on the route Vienna Schwechat - Dortmund Airport: Lufthansa (LH), Eurowings (EW).
There is no direct flight on the route honnan Vienna Schwechat   Dortmund Airport
The shortest flights with change last about 9h 50min.