Still in beta cycle, vicidial now has support for asterisk 1.8 version. I installed latest SVN of vicidial on my virtualbox. Installer now gives option for asterisk [1.2, 1.4, 1.8]. In fact that is where I found out that vicidial can run asterisk-1.8. Yes I am lazy. Now I have to find a client for testing 😉 while standing by to roll back and revert to proven asterisk 1.4 in no time.
Asterisk 1.8 would definately give better performance and security. However performance would not be significant. Main load lies on database side not dialer side. You would still need clustering when you hit one of those limits. Happy to see vicidial new features but that is another story. If any one of you intend to use asterisk 1.8 use either vanilla asterisk 1.8.21 and apply netboarder-cpd-1.8 patch here or get vici flavored asterisk 1.8.21-vici available on vicidial download page.