You can't install IE7 or 8 alongside 6 and no other "modern browser" has any kind of reasonable group policy management support or administrative control over updating or extension policies.
Virtualize what, how, and how does that help?
I can see terminal services running published IE6 for legacy apps with desktops having IE8 on them and doing that as a workaround to have side by side, but where would virtualisation help?
Assuming "You can't install IE7 or 8 alongside 6"
You can virtualise Windows XP on Windows 7 and run both browsers. This alleviates pain and provides for legacy apps that rely on 6.
Of course, infinitely better to just ditch IE completely and the dependent apps, and simply write that off [amortizing against future pain]