Have you considered bringing in a motorcycle dealership in on the deal?
You could use "demonstrator bikes" out of the shop that folks could rent for a few days, once they get a few thousand miles on them the dealer can still make money by selling the bikes off the showroom floor as "nearly new". In the meantime, they have made a profit off of the rentals that would make up for not selling new. Besides that, there is already a maintenance staff in place to keep the bikes in tip top shape.
Now that I have voiced this idea, I may look into it myself up here in the North
Just a thought
I think you'd be better off offering bikes for sale as part of the trip, you decide beforehand if you want to buy a bike, with an option to buy after. (You might be able to make money every time you sell a bike, also, since the dealer wouldn't have to do any work.) You're not going to get a nearly new price off of a rental bike. People beat the crap out of rentals. I do like the idea of having a dealership in on it though, Buying a bike at the onset could save you money on the price of the trip and you could have the bike shipped to your house at the end of the deal. Not so bad. I wonder what a dealership would want for renting out bikes, those you might need to buy.
There would be a lot of work that would go into pricing the cost of riding on a vintage bike, or any bike, though the money you make for rentals would be all yours, none would go to a dealer. You'd have to look at the cost of the maintenance schedule plus depreciation and insurance. Oil change every 2k miles or 2 months? Lots of oil and air filters, valve clearance adjustments, timing adjustments, sync-ing, leak issues. (lots of time invested) Depending on how many miles you really put on them you are going to have more frequent overhauls. There could actually be a premium for a vintage bike, but the cool factor would be unmistakable. Also, initial overhead would be lower. If you lose a bike, too, to an accident or 'old age' you'll have to work at getting a replacement. I think, though, that to do this type of an operation you'd need at least 2 guys unless you have some sort of shop in on it, like operator says. Help with the maintenance, coordination and legwork as well as (and I do think this is important) an additional person to carry gear in some sort of large(ish) vehicle behind. (Food, drinks, spares, luggage, injured people.)
I'm really getting into this idea. I think it would be a lot of fun to do. It'd be great to see it get off the ground.