Simple answer: there's no such thing as 100% secure. Even completely offline, encrypted networks can be compromised. You're probably fine though.
ProtonVPN
your question seems to have nothing to do with proton; please try restating it.
I'm using proton. proton is my VPN.
So my question is, for example, if I'm using public Wi-Fi, I'm safe behind my proton VPN wall?
passwords, banking app visits, embarrassing websites, is this all for-my-eyes-only now?
No, you are never 100% safely protected when using anything.
You only slightly increase your security rating by using something like a VPN when out on public WiFi, for example.
This is because someone could look over your shoulder and see your physical screen which would enable them to see the sites you visit. Someone could have a device that listens for your typing and can decipher your password from that. You could have downloaded malware before or during your visit to the coffee shop and that malware is now spying on you, despite your use of a VPN or not. If the website isn’t using https, your traffic is exposed to the VPN and anyone/any program listening to the packets leaving your computer.
A VPN is just one layer of security and even it can be a point of intrusion if you don’t keep up on the correct security updates, by either missing a crucial security update or allowing the application to update to a new update that has been discovered to contain a vulnerability that is being exposed.
when it comes to the internet, you can only be safer; NEVER fully safe w/o air gaping.
there was a recently discovered man-in-the-middle style vulnerability on vpn's, including proton's, so you're only safer if you're using a vpn connection from your android 14 phone until that gets fixed (if hasn't already been fixed)