Chickenpox vaccine reaction?

Morning! PN here - this is more likely related to the MMR component of the vaccine. It's a live vaccine which takes around 7-10 days to take effect in the body, so it's really common for side effects to manifest now and a mild rash is a common side effect of the MMR vaccine, as well as a short-lived fever which can pop up around now as well. If anything is worrying you at all or if they're seeming unwell to you then definitely contact your practice :)

https://www.nhs.uk/vaccinations/mmrv-vaccine/

It’s normal. Mine was covered in a red Dottie rash for 48 hours on about day eight.

Very normal, he could be more clingy and sleepy due to the heat too