A VPN is not a firewall by itself but only a virtual tunnel which has another public IP on the tunnel endpoint than your systems current public IP. A VPN by its own offers no protection against attacks, it protects only the communication between the tunnel endpoints against sniffing and manipulation.
Thus, the attacker would not need to hack your VPN in order to access your system. It only has to attack the tunnel endpoints IP address instead of your systems public IP address and the VPN will forward these attacks through the tunnel to your system.
Of course a VPN might be combined with a firewall or NAT and would offer some protection this way - but this is then a property of the firewall or NAT and not of the VPN.