Well, I can only speak for my country, but here, vaccination status is - like a lot of other information - private health information that is protected by privacy laws. You CAN always ask. People are, however, not obligated to respond. What you do with that answer if it's a therapist or doctor is up to you, but it is not mandatory for them to disclose such things.

I'll add a small note as to why I think it's kind of important: if you ask and then answer is no, and then you go "why not?" and they say they have an autoimmundisorder and can't take it, now it's suddenly not just a decision anymore, now you know a lot more about their health. And unlike only a free decision, not all Ts would want to share such a diagnosis with their clients.