Admittedly a late reply, but I was about to file my own bug report on this so I figured I'd just add to this one.
From what I can tell this bug has been in since one of the warlords patches. I've never had an AI sue for peace since then and I do play with vassal states. I posted a thread in another forum asking if anyone actually had an AI sue for peace in their game, and got only one reply over the weekend which was from someone who also hadn't had any peace requests.
I've spent many an hour digging through DLL to no avail, so I'm not sure what exactly is causing this bug. I ran some tests a while back and by doing some tweaks I got an AI to go through the entire doPeace function right up until it was supposed to send me the peace request, but it never came (I made an assert go off right before the beginDiplomacy function is called, and the assert did go off). I'm not sure if the beginDiplomacy function is available to modders, if it is I couldn't find it, so this may be something only Firaxis can fix.