The PIX can quite capably handle IPSec tnnels without a VAC, but if you're going to have a lot of tunnels or push a ton of data thru them, then you're better off with a VAC. There's no definate numbers cause it depends on how busy your PIX is, how many sessions are going thru it, etc, etc.
The data sheets are probably your best reference point though (there's one for the VAC specifically here too):
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...
[toc:faq]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 and UDP are I...