Example: phone is using VPN X and router is setup on VPN Y? Does traffic go through both VPN servers? Is there a conflict?
traffic will simply go through both VPNs. your internet speed will be crippled immensely. That’s assuming both VPNs are commercial VPNs (or personal). If for example, phone VPN is a work issued VPN, it might have some restrictions on some IPs or DNS from commercial VPNs and cause a conflict
If the router is tunneling all traffic through a VPN and providing subnet access to a device, then the device’s VPN tunnel is going to be nested inside the router’s VPN tunnel.
The device DNS would trump the router DNS. Your phone wouldn’t even see the router DNS.