Should I use Orbot with a proxy or VPN? If so should I place it before or after Orbot's connection to the Tor network?
Similar q/a linked below should also apply here. Not necessary imo. Although it might be more secure to use orbot's built-in transparent proxying (requires a rooted device) which will torrify additional system applications/etc that you or the browser connect with.
Updating for 2018: Orbot now allows transparent torrification without root on android and ios devices. However this is not always beneficial as it forces identifying information through Tor which can de-anonymize you at the exit. I personally now use a multi-hop vpn and Orbot simultaneously to allow the use of Orfox through Tor and everything else through vpn. This is possible on android as of v 7.0.