Yes, you should be able to access eBooks as well as databases and other Libraries subscription materials while using a VPN.
If you are using the WashU VPN, you should not be further prompted for your WUSTL Key when accessing eBooks and other subscriptions.
If you are having trouble accessing an ebook from off campus through the University VPN, try disconnecting from the VPN and log in to the libraries’ proxy server with your WUSTL key