I have just successfully setup a VPN Server behind a Linksys router, here is what I did.
I disabled "Block WAN Request"
I enabled "Multicast Pass Through"
I enabled "IPSec Pass Through"
I enabled "PPTP Pass Through"
I enabled "PPoE Pass Through"
I also forwarded the following TCP ports to the VPN server:
1723
500
50
51
UDP Ports 500 and 1701 need to be open when using L2TP with IPSec for encryption
The server is using a Bellsouth ADSL modem.
The client is on cable 800 miles away.