Apple continues to route IPv6 over the default route when connected to a VPN that is v4 only (at least for OpenVPN that is the case, I don't have any experience with others).
So that means if you have a v4 only VPN provider all IPv6 happily goes over the default route.
This is not surprising to me at all, traffic should follow the default route that is given. If you are privacy conscious you should already know how to disable IPv6... Honestly if it is a bug anywhere it is a bug in the VPN providers that they are not providing IPv6 services for their customers.
Thankfully with OpenVPN IPv6 setup is simple, and while it doesn't provide the privacy extensions like SLAAC (you can only get a single static IPv6 address on the other end of the tunnel), it does allow you to easily tunnel IPv6 traffic as well. I personally do this by pushing 2000::/3 across from my OpenVPN server.
On OS X 10.9 I don't have a way to disable IPv6 that I can find. In Network > Advanced > TCP/IP > Configure IPv6, there are only three choices: automatic, manual, link local. And as soon as I configure it with either manual or link local, the wifi connection changes from green to yellow and I no longer have even IPv4 Internet, although it remains identically configured as before. A while ago this popup had an Off option just like the Configure IPv4 menu does.
Doesn't work that way for me. On 10.9.5 setting the wifi connection's IPv6 setting to "Link-local only" causes me to be unable to ping6 other machines but IPv4 is unaffected.
So that means if you have a v4 only VPN provider all IPv6 happily goes over the default route.
This is not surprising to me at all, traffic should follow the default route that is given. If you are privacy conscious you should already know how to disable IPv6... Honestly if it is a bug anywhere it is a bug in the VPN providers that they are not providing IPv6 services for their customers.
Thankfully with OpenVPN IPv6 setup is simple, and while it doesn't provide the privacy extensions like SLAAC (you can only get a single static IPv6 address on the other end of the tunnel), it does allow you to easily tunnel IPv6 traffic as well. I personally do this by pushing 2000::/3 across from my OpenVPN server.