VPNs can be encrypted over what is called a "secure tunnel." Ask for more details about this "secure tunnel." It could be a setup where all tunnel traffic is encrypted with SSL (the most common encrypted setup) or one of several other choices, or it could be plaintext.
Even if it is encrypted, however, encryption will not prevent others from being able to access the network. They will not be able to access data being sent over it, but if they can access your computers, that's worthless. This can be negated if you use a secure setup in which computers can only be accessed by authorized users, even from within the network. If they can connect but can't get into any of the computers, it's no big deal. This, of course, depends on you keeping up-to-date with security patches, and still is not good as solid WPA protection.