Can you respond to an opponent playing a unit with a reaction spell?

Ruling: You cannot respond to an opponent playing a unit. Units leave the chain directly without a round of priority, unlike spells which linger on the chain.

Nuances:

• Units, gear, and abilities that Add resources all leave the chain immediately without allowing responses
• In the scenario described, Vayne would enter readied (not exhausted) because there is no opportunity to respond by removing your unit from the battlefield before Vayne enters