as3 flash project targetting flash player 10 error 1172

This is my first post so please excuse me if the answer is trivial.

I am creating a simple project where a movie clip contains a motion tween lasting 180 frames.

So I create a tween layer and create the animation.

I now create another layer and perform a stop(); on frame 30.

When I compile or test,  it produces loads of 1172 errors where common classes (like vector) cannot be found. 100 errors!

This is targeting flash player 10. when I target player 9 all works.

have things changed that dramatically with flash player 10?

what are the consequences of continuing with flash player 9? I prefer to fix the problem and enhance my knowledge.

Thanks guys.