Thank you for the opportunity to help you with your question!
vinaigrette - are technically called temporary emulsions. This means that the oil
and liquid, which naturally wouldn't join together, can be temporarily
interspersed within each other by rapid whisking or shaking. But because
there's nothing really holding the oil and water droplets together on chemical level, they eventually separate back out again.
Commercial salad dressings are usually made with a lot of additives
and emulsifying agents that keep the vinaigrette from separating. to separate the two you need to cool the mixture so that the olive oil can undergo the phases of separation.
Please let me know if you need any clarification. I'm always happy to answer your questions.