Since I am basically using MiaB as a personal email server, is it possible to use theVPS to be a VPN end point? or run Pi-Hole alongside?
MIAB is only supported on its own server. Though it may be possible. No one will be able to truly assist beyond best effort.
pi-hole definitely will NOT work along side MIAB as it will overwrite the /admin endpoint. (If they are on different servers it should still be fine however.)
In my home i have two raspberrys doing something similar. One runs pi-hole and in the other i have a mail server and various webpages. But i need a LAMP server to run this webs and MIAB runs nginx instead of apache, so i’m using iRedMail over raspbian (New versions of iRedMail also run only in Nginx). If you want run MIAB you will need ubuntu 18.04, but i don’t know if it runs in the ARM ubuntu version. In any case you need other machine to run MIAB.
Of course, you can use a desktop computer, install on it a virtualization software and run two virtual machines, one with pi-hole and the other with MIAB, but i don’t think that run MIAB and pi-hole over the same operating system (in the same machine or the same VM) is possible.