Assuming you brought up a PPTP vpn_wan interface and created a new vpn_lan network for your new SSID this could give you some hints. This is some adaptation from my isolated guest network and has not been tested, but should give you some points to work on.

/etc/config/dhcp

config dhcp 'vpn_lan' option start '100' option leasetime '12h' option limit '150' option interface 'vpn_lan'

/etc/config/firewall

config zone option name 'wan_vpn' option input 'REJECT' option output 'ACCEPT' option forward 'REJECT' option masq '1' option mtu_fix '1' option network 'wan_vpn'

config zone option name 'lan_vpn' option forward 'REJECT' option output 'ACCEPT' option input 'REJECT' option network 'lan_vpn'

config rule option target 'ACCEPT' option dest_port '67-68' option name 'VPN_DHCP' option src 'lan_vpn' option proto 'udp'

config rule option target 'ACCEPT' option proto 'tcp udp' option dest_port '53' option name 'VPN_DNS' option src 'lan'

config forwarding option dest 'vpn_wan' option src 'vpn_lan'