This feature which is available in Flash CS4 is a much more efficient Array. Unlike Array, you cannot place data in any given index. Each index must have a value. For example, you can't have data in Array[0] and Array[5] but nothing in between. The second parameter is a Boolean and it decides whether the Vector array can have more indexes added later.

//In this case, the Vector will only hold a DataType of <uint>
var numbers:Vector.<uint> = new Vector.<uint>(3, false);
numbers[0] = 1;
numbers[2] = 2;
numbers[3] = 3;
 
trace( numbers )