I wasn't aware of them either, but apparently they're just a way to handle multi-line text other than ML-text controls. RTB IRC Client uses them.
You have to make a messageVector object using:
new MessageVector(YourMessageVector);
You also need a GuiMessageVectorControl in a GUI, then you need to attach the messagevector object to the GUI control:
YourMessageVectorControl.attach(YourMessageVector);
Then you can use this to add lines:
YourMessageVector.pushBackLine("Hello this is a line that will appear in YourMessageVectorControl",0);
I'm not exactly sure what other methods there are (it has no proper dump method), but I found these:
pushBackLine("text",0); - add a line at the bottom
pushFrontLine("text",0); - add a line at the top
popBackLine() - remove the line at the bottom
popFrontLine() - remove the line at the top