Is your project to target Flash Player 10?
Require Flash Player version 10.0
That is likely the problem.
If this post answers your question or helps, please mark it as such.
I am targeting Flash 10.0. I'm only using mxmlc, not Flex Builder and I have "<target-player>10.0.0</target-player>" in my flex-config.xml.
If I wasn't targeting Flash 10, I believe I would get the message "The definition of base class Vector was not found." instead of any of the error messages I described.
It's a bug somewhere in Adobe-land, so file a bug and we'll figure it out. Just for yucks, try Array and see if you get the same errors. Vector should probably do what Array does
Flex SDK Developer
Adobe Systems Inc.
You create a Vector subclass by doing something like:
var v:Vector.<String> = new Vector.<String>();
The AsDoc should have been more clear and it looks like Vector should have been marked final, which is a bug. I filed it: https://bugs.adobe.com/jira/browse/FP-2483