I just love coding.
The second reason that initially I wanted custom solution, that can be flexible.
With my own project built from scratch I could introduce new any features much faster.
Well, your vpn had an interesting usecase(making traffic cheaper).
In russia everybody just uses some free browser extensions to bypass traffic filtering :(
Iceland is not free from censorship (and is far from the freedom of speech haven people seem to think it is).
The largest ISPs in Iceland were forced to block access to The Pirate Bay and Deildu.net (icelandic torrent site). These blocks were of course hugely ineffective and at most an annoyance.
And in russia blocks are not effective(everyone bypass them), but they also affect really peaceful websites. And it is only beginning, waiting for Great Russian Firewall.
I used OpenVPN.
Didn't have enough time to build custom client wrapper. So our customers used some openvpn client, or viscosity.
We had about 400 registred clients, of course not all were active and paying.
Getting paying customer is quite hard. Some people just register. Others register, pay, but do not start using. Some of them use service for 2 months, then leave. After 2 months they come back.
There different requirements, but most of people want super easy install process. But all of them have different network configurations, so even with simple client you will have to spent time helping people to start using service.
FWIW, that customer churn and on again off again usage pattern might not be anything to do with your service. I use inexpensive VPNs where I can register with an anonymous prepaid credit card and a "throw away" email address - I'll then use the service until the credit runs out and then switch to another inexpensive VPN provider. I think of it as "spreading my needle across as many haystacks as possible".
Real hackers and carders do not use low-end cheap VPN.
So our customers were mostly people who just want to bypass government blocks.