Nothin' like balancing the scales between 'not wanting to hurt your friend' and 'not wanting to be ineffectual'. So he does what he has to. He's assuming, of course, that Vegeta's just going to barrel past him. He gets out of the way, but uses the chance to get a real good look at where the gaps in his armor are, what tender bits he can hit.

He whistles again and this time the arrow zips in the armhole of his armor, tearing and ripping around under there. Not through his skin- it's too tough for that. Like striking stone, really. But it's still enough to sear and burn with its force, the smell of sizzled hair and trails of smoke billowing out of it.

He's going to make one less layer for Trunks to fight through, dammit.