Upvote:1
Yes, your return flight will be cancelled, unless you contact the ticketing carrier and rebook the flight to be an open jaw, YVR-MUC and DUB-MUC-YVR. This may change the fare, and will be subject to the rebooking terms of the fare class you purchased.
Had you booked these as two one-way flights, you would have been able to do what you propose without any harm, but that may have cost more than a return ticket did.