Will vpn interfere with any of my imessages

VPNs will interfere with a lot of things. Examples phone calls, 2FA text messages from banks, etc; and many other things. Unless you need a VPN for work connections, they are a waste of money, and the provider also has full access to any data you send and receive.

There is no reason to install a VPN on your phone unless your employer or school requires it to connect to their network.

The reason.....your phone is less secure with a VPN......despite what all the advertising hype might claim.

Even if you find a free VPN, it would not be worth the money.