I assume this is either a Pix or an ASA. What is the on the other side of the VPN Tunnel. Is the other side configure to accept connection from your NATTed Address to the 192.168.20.0/24 range or only specific IP Addresses.
From the problem description, it looks like the remote side is configured for specific IPSEC Acl or permitting only specific traffic to hit the servers.
If they have their side configured correctly, then you need to configure the mirror acl for the interesting traffic on the ASA, bring up the tunnel, and look at the outputs of "show crypto ipsec sa" for encrypts/decrypts to see if the ASA is encrypting the packets and sending it to the remote peer or not.
If the remote side is configured for the class C range, can you reconfigure the ASA to mirror the remote ACL, bring up the tunnel and post the outputs of "show cry is sa" and "show crypto ipsec sa".
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...