One thing that comes to mind is timeshares. You have a monthly payment and get specific # of weeks to use. I have found that most of them you need to book way in advance and their websites are crap. Building a solution to solve that by using reminders and/or other systems (voting) might be useful. - My 2 cents.
Why not just deposit that money into a preferably interest-earning account that you control? At the end of a year, you will have the cash you need for a vacation, plus some extra from interest.
> A thought came to mind about subscription based vacation site. You can deposit money throughout the year and then 2-3 times a year plan a vacation. Create a playlist of vacations.
It sounds like a generally interesting model but it seems like it would be faced with being squeezed uncomfortably between simple direct vacation rentals and vacation ownership offerings (which are pretty diverse.)
Who is your market and why is this better for them?