Can anyone tell me how to access to the 192.168.2.1 web interface of a Vigor 120 modem that is connected via WAN2 of a 2820 router and setup in PPoE mode?

I have the exact same question.

Anyone know the answer?

i don't think can be accessed at pppoe mode.
i tried mine (but with a 2850n) and nothing even with telnet
EDIT
It can access the 120 v2 interface if will not set up at pppoe but the 120 v2 will not let me with any other way just pppoe

It's because PPPoE mode doesn't pass any TCP/IP traffic through the WAN2 port, what you need to do is put the PPPoE server (the vigor 120) on your LAN network then connect the LAN into the WAN2 port, this will give you access to the 120 if you set it so it's on your router's subnet and the PPPoE traffic going over the LAN through to the WAN2 port shouldn't disrupt the local network at all