Say you have hotspot on your phone and laptop connected to it. Laptop has VPN, is it safe or does the phone need to have VPN but then laptop's VPN is redundant?
A VPN creates an encrypted tunnel that you can use on an otherwise unencrypted line to obfuscate your web traffic. If your laptop has VPN and is connecting to the internet through your phone, then the cell carrier will probably know that you're using a VPN on your laptop, and they can probably tell some things about the traffic. For instance, even if you're using a VPN, it's easy to tell the difference between filesharing and web browsing. That said, they probably won't be able to know what websites you're going to or what files you're sharing. If you want all of your data to be encrypted when using the phone as a hotspot, then putting a VPN on your phone could make having a VPN on your other devices redundant. EDIT: A few other things to keep in mind. Using a VPN on your phone will drain its battery more quickly. Using a VPN on any device will increase overhead. You could use up to 10% more data when using a VPN vs. not using a VPN. Lastly, using a VPN on your phone could reduce your overall throughput since the processor on your phone doesn't have efficient instruction sets or the horsepower to encrypt a lot of data quickly.
The device that is connecting to the VPN service has to encrypt the outbound traffic and decrypt the inbound traffic. The VPN service does the same thing at the exit node for the VPN. It will encrypt the data that it's sending to your device and decrypt the data that it's sending out to the internet. If you're using your phone as the VPN endpoint, then your phone will be tasked with encrypting and decrypting all the traffic. If you're using a laptop connected to a cellular hotspot, but the laptop has the VPN service running, then the phone won't have to do the encryption/decryption, but you also won't have all the data from the phone going through the VPN. In that scenario, only the data from your laptop would be encrypted.