I know this is a manga and devil fruit abilities aren't designed to make sense as much as them just being cool, but....
So Vanderdecken's ability allows him to touch something, then anything he throws will accelerate to that target, unless blocked. if he touches anything else, that thing will overwrite his old target and become the new target (as evidenced by him never even washing one of his hands and wearing a glove.
The problem is, why doesn't whatever he throws just become the target? He clearly doesn't have any control over wether or not something he touches becomes his target, so shouldn't the arrows, knife, or giant axe have home his target as soon as he picked it up to throw it?
Furthermore, how does a glove protect him from overwriting his target? Wouldn't the glove just become a target?
Also, I don't see why he wouldn't wash his hands. If he does wash his hands, would the water he touched become the target? If so, what would he do about the rain, or possibly even the air surrounding his hand at all times?