Simple skiplagging, hidden city ticketing, done every day. (as long as this is an one-way ticket / skip segment is the last ticket segment)
Yes, you can do this with little / no risk -- if you are not a repeat offender
In irrops you could be routed to ZRH or MUC. You can decline but options can be limited for awards and it must be a solution getting you to VIE. Explaining you are willing to just go to FRA may get strange looks.
UA thread: Is Hidden City Ticketing (HCT) illegal, immoral or just part of the game?
BTW, VIE is worth visiting