I think this cannot be set at the VPN client side. It has to be set at the VPN gateway (PIX< Router or VPN concentrator) end. When the policies are downloaded from the gateway, the split-DNS details are also dowloaded to the client. If you are using VPN concentrator as the VPN Gateway, here is the document that explains on how to configure split-DNS.
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...