I am not a vision or dental professional so this is just my opinion (and sort of what I do myself anyway).
Reschedule the cleaning, don't bother with vision or dental gap coverage. You are talking less than two months until you get 100% paid for coverage, right? That would be a no brainer for me personally.
It's a gamble, but likely the odds are in your favor. You're only talking a few months and only two out of three people in your family would actually need anything, and keep in mind if it's a medical emergency that involves your teeth, it is likely covered under medical insurance (check your policy to confirm).
I always figure that it's worth the gamble myself and we've been self insuring for dental/vision for the last 4 years. Husband even chipped half a tooth in the front off and our dentist got him in that next day, it still only cost us $175 out of pocket to basically rebuild a front incisor (we have an EXCELLENT dentist).
And I do a cleaning/checkup around every 8-10 months. My hygienist told me that some folks have excellent teeth/gums and can get away with an extended schedule and she'd be sure to tell me if there was ever any issues and need to come in more often, but you don't have to stick to a 6 month rotation if you are really good at your oral health and have had great checkups for years. Going in at 7 months one time in your dental checkup routine would be nothing unless you have severe issues with your gums or something.
And as far as vision, as long as you've had decent checkups within the last year, you likely could go to a 2 year rotation on eye exams as well. They are so cheap and easy to get out of pocket, but if you really cared, you could just enroll every other year (opt out this coming year, enroll next year).