Yes, connect Slate WAN to Fritzbox LAN.
Devices on Slate Plus LAN/WLAN should be able to use the VPN client simply by configuring the VPN client in the Slate VPN admin page.
To allow devices on Fritzbox LAN to talk with Slate Plus LAN see the recipe at Access devices behind GL-SFT1200 (Opal) from different local subnet. I think this may also allow the Slate admin panel to be accessed from the Fritzbox LAN.