What does it have to do with app store? Insecure apps which might not respect server TLS certificates / settings or communicate over plain HTTP will be insecure to use over a VPN as well. A VPN is not an alternative to not using proper TLS validation.
You specifically mentioned TLS being enforced in mobile apps. For non-mobile apps such an enforcement either happens through an app store vetting process or the operating system restricting access to non-secure API calls.
I also didn't say a VPN is an alternative to proper TLS validation. It just prevents public WiFi networks from trying to intercept (improperly validated) connections.
I said "mobile apps" to exclude browsers which do similar validation anyways. And it is the same process for mobile apps, only apps designed in an insecure manner (to choose to ignore cert warnings, use custom TLS clients etc) would fail validation and there is no reason to use such apps, it does not matter whether you use a VPN or public-wifi.