Yowza. Your diagram is well beyond my understanding. If I follow, there are four high end routers in the mix; you have two DHCP servers at work at the primary home, and no dhcp server at the vacation home. I don’t follow how anything in the vacation home VPN router wireless connections get anything in the 2.x range if the LAN DHCP server is disabled there, or how they connect to anything. It also looks like nothing would work at the remote locations if the tunnel is down, even enough to diagnose the problem. But as I say, beyond my ken.
As a stab at it, you would set up a OpenVPN client on the Slate configured exactly in the same way as the vacation open VPN router client is set up, and set it to connect. Disable the DHCP server there. When you are at the Airbnb, under Repeater, click scan and join the wifi there. That gives the Slate an internet connection, and the client should then connect. If it works for the vacation home, then I guess the slate would work as well. You could also give the Slate different certificates (a different CN), and then the primary home VPN server could configure a different scheme if you needed it.
I would have done this with TUN: the primary home 1900 running DHCP for a network at 192.168.50.xx and a OpenVPN server configured for access to the LAN and DDNS; then a vacation home 1900 running DHCP for a network at 192.168.51.xx and an OpenVPN client configured to connect to the primary home; then the Slate running DHCP for a network at 192.168.8.xx and an OpenVPN client configured to connect to the primary home. Actually, this is exactly what I’ve done except with two Asus routers
At least then, if there is a problem you can have normal internet access anywhere and diagnose the problem. The vacation home and the slate could reach everything on the primary home LAN. You could use policy rules for certain devices to only connect over the LAN while others have split tunnels, and you don’t limit your speeds to the uplink at home.