as3-signals looks nice, but seems like a lot of code to pull in for something so simple. Doing so would be fine if developing an app, but it can be annoying for a library to pull in things you don't want to use.
EventDispatcher looks not all that nice, plus seems to be overkill.
Vector is very simple and you can "really" add/remove listeners just fine:
event.splice(event.indexOf(func), 1);
That Vector's API sucks isn't my fault. 😉 Possibly it would be ok to create a simple event listener class that has a Vector and add/remove methods to make the API a bit nicer.