Question

As we know in vacuum the gravitational force doesn't work so why a feather and iron ball left from a height (h) came down in vacuum .

Answer 1

To answer your question you alreadyunderstand that they all have the same acceleration acting on them but they don't fall at the same time because due to the low weight of the featherand it's surface area it would be subjected to significant air resistance which slow down its fall…if you do the experiment in open air ...