Wei Wuxian leads him towards a secluded area that is surrounded by trees so that it is quiet and they can be alone. He wants to be able to have just them for this final moment of the wedding day. He tilts his head up to smile at Lan Wangji.

"Are you ready to release it? You have your wishes and vows ready?"

He had promised to not only share his wish with Lan Wangji but to also write vows that they would have as promises to each other. A promise is something they will actually uphold whereas a wish is something they hope for. Both are important.