It should be fine because much of the benefit of IFing comes from simply not consuming carbs and protein, that's what changes the genes expression and induces the autophagy.

Someone might come along and point out something that might be wrong with vitamin d, but you are still going to retain most if not all of the benefit.

D3 just needs to be combined with fat in order to be absorbed properly. So if you choose to do this, maybe take it with a little oil or put some cream in your coffee.

This is exactly what I do almost daily. I wake up, take 200mg of L-theanine, 500mg of L-tyrosine, and 2,000-6,000iu of D3 depending on the weather. I then have a cup of black tea with heavy cream and cinnamon.

I haven't noticed and ill effects, and I've been getting leaner and leaner over the past 6 months that I've been doing this regularly.

Yes & Yes.   If you are taking D3 in an oil-base, you don't need to have it with food, IME.             

cinnamon lowers blood sugar levels as well, so you'd be fine with that!;o)

I don't know about the coffee (I'm obsessed with teas), but I would suggest having master cleanser in the morning instead if you want better IF benefits... When you're fasting, you should only drink liquids. It doesn't matter what you put in your liquids, as long as they dissolve. I use my Magic Bullet to achieve this, usually, or I mix it in the master cleanser.

If your D3 is in a gel form (liquids are more easily absorbed, tablets have bad absorption rates), you can just empty it into your shake/drink in the morning. That way, your GI tract doesn't have to work to digest anything (just absorbs) and the cleansing continues!

The master cleanser is intended to get your good ol', healthy digestive juices moving for some good old fashioned cleansing, the way our ancestors did it. Word is, once they realized how good they felt when they were starving, they started doing it during times of food abundance, full-aware of the benefits of this type of cleanse, which is made possible by the digestive juices doing their thing and letting the GI tract take a break, while the body catches up on its homework. The lemon and cayenne pepper stimulate stomach acid production. I personally don't use the maple syrup.

Fwiw, when I IF, I just go without my D3. Since vitamin D is fat-soluble, it persists in the body for long periods of time, so there's really no need to take it every single day, or at a particular time. e.g. some physicians prescribe megadoses (50,000+ IUs) of D3 once per week for severely deficient patients, and once-weekly doses appear to be effective at bringing up D levels. If I'm planning to do a 24-hour IF, I'll just take somewhat more D3 during the 2 surrounding meals, rather than taking it while fasting. D3 is optimally absorbed when taken with fat, and fat eaten during a fast breaks the fast...