Using the Czech-speaking e-shop of Czech railways
Start at [main page of the eshop]. First, you have to register. TBD
Buying a ticket to Vienna
In the menu on the left side of the page, click on "Rakousko" (Austria) under "International offerings" to get to [section for tickets to Vienna].
Select "Brno hl.n.", "Wien" (Vienna), date, time, and hit "Vyhledat" (Search).
Then select the connection and hit "Zakoupit..." (Buy...) next to it.
Leave the parameters on the next page as they are, and "Zobrazit jizdni doklad" (Display the ticket).
Then "Vlozit do kosiku" (Add to the shopping cart).
From that point Google Maps cannot help you, but if you win over the beastm it will teleport you to the FUDCon... ;-)
WARNING: Thought the e-shop technically sells tickets for both directions independently, the ticket for Vienna->Brno is not accepted in the train unless you have a corresponding ticket to Vienna, appropriately stamped to prove that you have indeed already consumed the first half of the return ticket.