Simple, my friends. If we assume for a moment that nobody in the house is unaccounted for (after all, it would be quite difficult to sneak into the home of a crimefighter like Vayne, as she would have a ward security system), then the only reasonable answer is that Vayne herself is the perpetrator of this grisly deed.
Consider the following. Why would she have her maid clean the scene of the crime? Such action would destroy precious evidence; her exact reasoning! Asserting a desire to avoid alarming her guests was a clever ruse indeed, but her very downfall, for why would one seeking to avoid causing alarm inform the guests of the murder and it's pending investigation? The answer is simple: this investigation is nothing more than her personal alibi!
The puncture wounds in the victim suggest a thin stabbing weapon was used, and she's already noted a lack of any void energies or gunshot residue. She says that these puncture wounds perforated the victim, meaning that they've gone through and through; katarina's daggers wouldn't hve that kind of force behind them, but Id say a crossbow bolt certainly fits that description, wouldn't you? Finally, how is it that miss Vayne is so clear on all the detail so of this tragedy? The hostess of a party is supposed to be idly mingling with her guests, not tracking and recording their every move in a pre-emptive investigatory effort.
Lady Vayne, I'm afraid the evidence against you is nigh insurmountable. Officer Caitlyn, take her away...