What I know is you can actually book the flight (reserve the flight) but not purchase/buying it yet. This booking can be shown when you are applying for a Schengen visa. You can always go to travel agency about it. Some agency just ask a small amount so you can have this flight booked. After you get the visa, you can go back to them and pay the booking to confirm it.. In case the visa is not granted, you can call the agency and cancel it.
Reference: Learned it from a friend who work in a travel agency before. I was applying for a schengen visa too for vacation but decided to save up for my fiancee visa