After the Principium, when the vectigal is tested for sincerity and willingness to bond, the potential domini are evaluated as well, and anyone who was unwilling to bond would be able to subtly make that clear. A lack of suitable candidates for dominus or a direct refusal from one of them would weaken the vectigal, and when he was able to complete the bond he would be more dependent on his dominus and more slavishly obedient to him.
--> Desire for clarification. Would "subtly making clear" that you don't want to bond benefit the vectigal in some way? Would it weaken him or her less than a direct refusal? That's the sense I got out of the above section, but then I wonder, could Harry "subtly" refuse at this point (could anyone, really?) or would it really amount to a direct refusal? Or is Harry and Draco's case really a special one as they've been squashed into this unsubtle deadline, and thus there's no option for the subtle refusal that there normally would be?