It's better to pass the exam on which version you have much working experience or you are planning to upgrade your infra to the latest version. Here find the exam details and topics covered:
In version 8, you will find some new concepts like Tanzu, DPU, vSAN ESA etc.
For your first question, you currently have the choice to take your training on vSphere 7 or vSphere 8, and to pass an exam on vSphere 7 or vSphere 8, my suggestion is to do whichever you will find best for you based on what version you already work with or will be working with.
If you want to be able to tell people you are trained and certified on the latest vSphere version, do your training and exam on vSphere 8, your certification will still only state VCP-DCV 2023 but you can prove that you met the requirements with vSphere 8 training and by passing a vSphere 8 exam if needed.
A screenshot from Certification Manager will show the specific exams you have passed.
Please stop saying VCP7 and VCP8 😉