Vambrace is a bad item. For several reasons. It is given to late. It is just an item you get until you replace the null, bracers and wraith band. Theses items are meant to help you survive in the laning phase but this item is gotten well past the games middle development, and the laning phase is the beginning phase. Additionally it only combines like items: two bracers, two nulls, or two wraithbands. And that item setup is not always viable. Some times you may need a null and a bracer, or a wraith band and a null. Or a wraith band and a bracer. Depending on your hero you have and what is happening in the lane. Vambrace is a bad item. Additionally, it fulfills the purpose of kaya, yasha and sange.
The only reason I would get this is if I wanted to have a sange, yasha or kaya updrade. Those items are built with the null, bracer idea without using them directly.