is possible somehow setup the main router (not openwrt) that a specific IP device connected, goest to the openwrt router first and use the tunnel vpn and then go back to the main router and continue go out to internet? to avoid using the AP openwrt wifi for that device.
using static routing?
I'm not sure I totally understand what you're asking here, but if I'm getting your thought, no, this is not possible.
Your main router would need to have a system to provide a different gateway to the device in question... most routers do not have a way to do this... it is possible with RADIUS based systems, but that is seriously overkill. Static routing is not going to help you here, although Policy Based Routing could be useful.... you'd need to connect through your OpenWrt device though, so you don't fix the issue of that being low performance even without using the VPN.
From there, your choice is to either send the traffic through the VPN tunnel or via the main router. When traffic goes through the VPN, it is not possible to send the traffic back to the main router out to the internet (this is the part of your question that I don't totally understand, but the device using the VPN isn't even aware that the main router exists because of the tunnel) except insofar as the VPN tunnel itself uses the main router as its gateway.