For an easy, inexpensive and quick solution I'd recommend a Fiamm 130dB Freeway Blaster, low-tone trumpet-- that's what I replaced the stock horn on my F with. You can get it at auto parts stores or Amazon and it'll bolt right up in place of the stock VRSC horn-- no wiring modifications required.
If you are a little more creative, you might consider a Denali Soundbomb Mini. It comes in both one and two piece versions for mounting flexibility. I've never seen or heard one being installed on a VRSC before, but don't see why it wouldn't work with a little thought on how to mount it.
It's fairly compact, but Mother of God!......... I installed one on my GS recently, and when I hit the horn button, both myself and my buddy literally jumped in surprise; it sounded like the Great Northern was rolling through the garage. That thing is LOUD.
If the cager next to you (or in front of you, or behind you, or possibly on the next street over) doesn't hear you-- they're stone deaf and it's a lost cause. Denali markets it as over four times louder than a stock motorcycle horn.