Off late, I have seen a lot of implementations using VLAN for separating the zones and using the same switch. As far as you have tight control to the the device, strict change control process, auditing, Best Practices, up to date software updates on Security Advisories, etc, you should be fine using VLANs. Also, one important factor that is going to drive your decision is the companies "Security Policy".
With that said, below are some white papers that you might find useful.
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...