Yes, doing your own leak prevention is best. Unless the VPN provides its own IPv6 address, disable IPv6 everywhere you can, and block it with firewall rules. Use firewall rules to allow connections on physical interface only to VPN server. Restrict everything else to the VPN tunnel. And make sure that you're using VPN-assigned DNS server(s) through the VPN tunnel.
But the six totally leak-free Windows VPN clients do that. FrootVPN and Perfect Privacy also provide their own IPv6 addresses.
I'm sure that others that I haven't tested do too. I'd bet on BolehVPN and Insorg, for example.
I'm ready to test more VPNs. For ones that I've tested, see <;. Results for OSX clients will go live soon.
Anyway, please recommend others to test or retest. Currently, I'm planning to test BolehVPN, Insorg, Windscribe and Witopia. I'll also retest NordVPN, because it was IPv4 leak-free, and AirVPN in OSX.