Explaining switch of research interests in Statement of Purpose
You don't have to convince the admission committee about the switch.

You do have to convince the admission committee that you are able to complete a PhD in Computer Vision.

Work out what skills will be needed, and present your evidence that you have those skills.

Work out what knowledge will be needed as a prerequisite, and present your evidence that you have that knowledge.

Work out what the other pre-requisities are (funding, contributing to the internal life of the department, whatever), and present the evidence for that too.