Get investors to be able to visualize as much as possible. Tell a story. Focus on the solution it provides. Make your story relatable if you can. If you can get their buy-in on all these things (the problem, solution, common ground, good delivery of the message, etc) the prototype becomes less important at this stage of the game and all of those other variables will overshadow the thing you don't yet have. In your story/explanation, provide as much visual detail as possible--while still keeping it simple. Remember your ABC's and 1-2-3's.

You probably won't be meeting with VC's with just a prototype (unless you're in certain industries). You're better off meeting with angel groups and allowing them to see and use your prototype. Your prototype won't necessarily increase venture capital but it may increase your valuation if the rest of your business fundamentals are solid.

You may negotiate with your investors and they may increase the amount if the prototype is really good.