Exegesis:

ti oun poiēsei autois ho kurios tou ampelōnos ‘what then will the owner of the vineyard do to them?’ This question interrupts the parable since it is addressed to the listeners. No answer from them is expected or given and the question serves to enliven the parable and focus attention on its main point. It is in the future tense as if Jesus and the listeners witnesses of the events.

Translation:

Cast him out … and killed him: the victim is cast or pushed out of the vineyard while still alive, and then killed outside.

What then will the owner … do? The force of this question may have to be brought out by saying, ‘what then do you expect the owner … will do…?,’ or as a statement, ‘you all, or, we (inclus.) all, know what the owner … will do, of course….’ Then, or, ‘therefore,’ ‘so,’ indicates that the question draws the inference from the parable, and as such marks its end. If this is still felt to be too abrupt a transition, one may add, “Jesus asked” (Good News Translation), or, ‘thereupon Jesus said.’

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.