i don't 'yet' use a VPN
but it would depend on whether the security starts in your PC or at the VPN server
if the data is encrypted inside your PC or at the VPN server
even if the data is encrypted from your browser to the vpn server
the data between your PC and the public wifi is not
every device connected to the public wifi is logging in with the same password "if a password is even needed"
which allows any evil pert, to monitor your signal traffic and attempt to break the encryption key of your VPN