I would not directly apply vinegar. It has a rather low pH of around 3 and can kill roots and such if it makes direct contact. In fact we use vinegar to kill moss on tree trunks. You might consider diluting it 10:1 or so with water when you water so that you are applying a somewhat acidic water. I acidify my water (pH 8) with phosphoric acid to bring it down to about 6.5. Acetic acid (vinegar) has been recommended since the phosphorous can tie up the iron in the soil so I may make a change.
The iron sulfate can be applied to surface and watered in, but I would probably scratch it in or cover with your composed bark since it will dissolve faster if kept moist. I could also be dissolved in water and applied that way, but that will stain anything you apply it to.