I have a ppp interface ppp1 created by the following command:

pppd pty "pptp 1.2.3.4 --nolaunchpppd" user vpnuser password vpnpassword lock noauth nobsdcomp nodeflate nodetach

It is verified to work. My tinyproxy config:

Port 8888
Listen 70.80.90.100 #public ip of my server
Timeout 600
bind 10.0.77.11 #inet addr of ppp1 interface

The proxy starts, however when I use it to connect the remote client (e.g. web browser) I get the public IP of my server (70.80.90.100) which isn't what I expect (it would be expected to receive 1.2.3.4 (PPTP VPN server IP address) as the public IP).
Am I missing anything or is this a software error? I assumed the bind to interface feature would allow to route traffic from proxy clients through the specified interface without routing all traffic of the server through that interface (ppp1 in the example).