I can't think of any situation that would call for exhausting my treasury on every vassal. A lot of the poorer regions might be satisfied with 20, but some of the wealthier ones won't take anything less than 100 or two. How will the "Gift All" command decide who to gift with your limited funds? Will it blow it all on happy bishops, or will it gift the entire thing to one wealthy grand mayor and leave your unhappy dukes with nothing?
Pretty simple to just go into your character sheet, click on vassals, and arrange by lowest happiness. Even easier, ignoring their happiness until they start factionalizing, and simply bribing off the largest contributors.
On the character sheet there should be a way to select multiple characters (by clicking a box right next to it) and then sending gifts (or any other interaction really...) to all of them at once.