Well, the purpose of Orbot is very similar to that VPN’s primary purpose – to maintain internet users’ online anonymity and to evade firewalls. Like VPN, it can also be used to spoof geo-location.
but it's not as secure as VPN and it's very slow – because your data is randomly bounced through a number of nodes, each of which could be anywhere in the world, using Orbot can be painfully slow.
Also android has a built in VPN client and you don't have to download any third party apps that it's a plus for VPN, in my opinion.