VPN on SSID #2 (OpenWRT)

Your problem lies with the routing table. A VPN modifies the routing table: since the table is unique, a VPN modifies the routing table for anyone who is connected to your router.

However, Linux allows the possibility to have multiple routing tables, and to apply one of them depending on the properties of the LAN user: it is called policy routing. Luckily for you, OpenWRT does implement policy routing: you can find a description here; even more luckily, it describes as an example exactly your case, i.e. a user wishing to route some LAN clients through a VPN, while leaving other clients outside the VPN.

For this to work, you will have to have users attached to Home_VPN to belong to a net different from those of HOME. For instance, you will have to instruct your dhcp program to give addresses in the range 192.168.51.0/24 to clients of HOME_VPN, and addresses in the range 192.168.77.0/24 to clients of the other wifi network. If you do so, the IP address of the router interface on HOME has to be 192.168.77.1, that on HOME_VPN must instead be 192.168.51.1.

You may then establish connectivity between the two separate networks by means of the following rules to add to the routing tables:

  ip route add 192.168.51.0/24 dev IF1
  ip route add 192.168.77.0/24 dev IF2

where IF1 and IF2 are then names of the wifi interfaces on the HOME_VPN and HOME wifi networks, respectively.