What you are saying is reductionist and simply not as clear cut as you make it out to be. Yes, there is no such thing as 100% privacy and security whenever doing anything on the internet by its very nature. Even the Tor network is open to attack.
However, there is a big, big BIG difference between accessing sites and other resources on the internet from an IP address that is directly associated with you (e.g. from your ISP) while using your ISP's or other public DNS servers, and using a VPN with strong encryption that makes reasonable assurances that they do not track you or keep logs. With the former, your ISP, the DNS servers, and anyone listening in on your network can see everything that you're doing. Even if your traffic is encrypted, they know which hostnames you're accessing and when. With the latter, it would take an extraordinary amount of resources and/or a subpoena from a government to correlate your private traffic with what's actually exiting the VPN.
Furthermore, several VPNs have even had requests for their logs made by governments that they were legally compelled to follow and they could not provide it because they did not have them. So obviously there are VPNs out there that actually do protect your privacy, data, and traffic history.
At the end of the day, you have to make a pragmatic choice. There are no 100% guarantees, but some choices provide significantly better privacy than others. It's disingenuous to suggest otherwise.
That all said,
"To answer your question directly, you can't do that. That feature would have to be built into the VPN client."
...is the sort of answer I was looking for. Thank you. My continuing hope is that there's some hidden feature of iOS that provides this that I'm not presently aware of, or perhaps a third party app that finagles a way of doing what I want.
