He is not going to stab Jiang Wanyin. It would cause more problems than it solved. Jiang Wanyin does not know that "Mo Xuanyu" is Wei Ying or that he practiced demonic cultivation at Mo Village. Probably he does not know these things; Lan Wangji should really know better than to set lower bounds on Wei Ying's ability to get in trouble.
He considers his options.
He could attempt to speak with Jiang Wanyin. Ugh. Terrible. He doesn't even have Sizhui or Jingyi to do his part of the speaking for him. And what would he even say.
He could...continue to stand here just barely out of sight until one or the other party left. Unacceptable; that would leave Wei Ying far too close to alone with Jiang Wanyin for an indefinite period of time. No.
He could walk in, grab Wei Ying by the wrist, and physically drag him away. Hm. On the one hand, Wei Ying isn't likely to be pleased with being dragged off with no explanation. On the other hand, it's not like Jiang Wanyin understands or wants or expects to understand Lan Wangji any more than the reverse, and probably this would not raise his suspicions particularly. And it would involve Wei Ying not being in the same place as Jiang Wanyin, immediately.
Well, that settles it.