VPN Encryption: If you are connected to the VPN on your home network, then any data that flows over the Internet is entirely encrypted whether you are connected to the Internet either wired or wirelessly. This means that if your Local ISP was trying to see what you are browsing or downloading, they would not be able to see your data.
Wireless Network Security: If you leave your wireless network open or insecure, it does not matter whether Internet traffic is encrypted, because someone can directly see what you are doing on your network, or even worse, can go in and put a wireless password on your network, locking you out. They may also be able to access folders and files on your computer without your control.
Best thing to do is protect your wireless network with a WPA secured password along with your VPN connection.