[If this was a button-up shirt, Van would just drape it over Luke's shoulders himself; but it is not, and he really doesn't know if he'd be successful forcing it over the head of a recalcitrant replica. So instead he just holds it out, in a manner that very sternly suggests taking it would be the best course of action for everyone involved.

For the sake of ingratiating himself to Asch again (and perhaps the replica too, as a bonus), he is going to explain something to Luke. He is going to explain it calmly. He is going to explain it clearly. He is going to explain it framed as the gentle guidance of a person that knows more and better. And he is going to explain it in the same soothing teaching voice he's been using for the last seven years, with the mild addition of bored exasperation.]

I don't know why Asch allows you to stay with him. Where you are concerned, his feelings have always been... extreme. But I do know that if you wish to keep staying with him, you shouldn't upset him. He thinks of it as his body and his face. Walking around town nearly naked will not endear you to him. [So you want to stay with Asch, replica? Do you see how Van is helping you? ...For now.]

Put the shirt on, Luke.