Is there any level of luck that is able to be tested in which it gives you the optimal re-roll?

Having no luck wouldn't re-roll an original vanity drop, but it wouldn't re-roll an Elite to a Vanity.

Obviously it depends on where you are farming, but say that all loot is droppable, wouldn't having some luck percentage be better to re-roll for a vanity versus none?

The only situation in which having no luck seems helpful is where the drop table contains only a small amount of possible levels.