It doesn't trace because when using "myVector[0] = ", you're actually using the getter, not the setter (You GET the vector to then SET one of it s values).
I would only implement a getter.
Instead of myVect[0] = 1 you should do _myVect[0] = 1 or implement a public method like so:
public function updateVectorAt(value:int, index:int):void
{
_myVect[index] = value;
// do the other stuff here...
}