Is Orbot VPN a real substitute for Tor on Android or not?
They seem to advertise a User -> VPN -> Tor setup which you should be able to do with free software apps instead of installing anything from them.
Orbot is Tor for Android however this site is suspicious and unaffiliated with the Guardian Project who are the authors behind Orbot. You should only get the apk through trusted channels. As for layering setups with VPNs choose any service that supports an OpenVPN client app instead of installing a proprietary app that could be spying/data-mining your activities.
Also remember that hiding Tor usage from your ISP is pointless for many reasons.