Been driving my JPS (same body as Vintage) for eight years with only a tonneau cover. Never driven the car with top up - only with a tonneau. One snap at corner of dashboard, one snap on body behind the door, no snaps on door. After all these years, I have to admit some paint scuffing is just now starting to show at mid-point on the passenger door due to "tonneau-flapping". Nothing a bit of cleaner wax won't eliminate, but I am concerned about the long term effect. I try to keep a lot of wax on that area now.
Next time, yeah, a snap in the middle of the door would be inviting.
By the way, If you have a custom tonneau made, be sure to get it tucked generously into the windshield base. My "standard" full tonneau lacks at least about an inch of covering the whole dash at that point. Maybe it shrunk, but the snaps still fit fine?