The problem with Orbot's VPN mode is it doesn't support chaining with another (real) VPN, since Android only natively permits one VPN at a time. This is a problem for users in places where the Tor network is blocked (and Tor Bridges / obfuscated relays are, too, or are unreliable), because you need an initial hop outside of the Great Firewall in order to reliably link to Tor.