can i mention a mini pupillage on vac scheme application

Mentioned whatever you want. I'd lump all of your open days and mini-pupillages into a single section on work experience. The only exception to this is for firms who specifically ask you to mention their open day in the work experience section. I would add these separately.

I would say mention it. It shows that you have considered a career at the bar and you could use it to justify that you have reached the conclusion that it is not for you (if that is the case). Firms and Chambers understand that you need to get a feel for both to know what is right for you and which path you want to take, if any, into the legal profession.