Is it essential to apply Vit. A & C on the skin before microneedling on darker skin tones to prevent hyperpigmentation occurring

Microneedling does not usually cause post inflammatory hyperpigmentation as the delivery mechanism is not heat. If we were to prep darker skin tones prior to microneedling, we would use a compounded topical medication with Hydroquinone and Retin-A nightly for about 3-4 weeks prior to the start of the procedure.

Usually micro-needling and RF do no cause post inflammatory hyper pigmentation. I usually prep the skin of my patients who are planning to get Mixto Co2 laser with bleaching cream and Retin A cream for about 2 weeks prior to the procedure. I have performed many Micro-needling and Dermaroller procedures without Hydroquinone or Vit C prepping and have not experienced any hyper-pigmentation. 
Having said that inform your doctor if you have very delicate and sensitive skin that turns brown easily after a pimple or minor trauma. Pre- treatment with fading creams and Vitamin C won't hurt. It may make the skin more sensitive but your doctor can adjust the settings for you.

To prevent post inflammatory hyperpigmentation on darker skin types, we prep the skin 4 weeks prior to  the scheduled microneedling visit with Hydroquinone and tretinoin (ZO Medical Melamin-C, Melamix and Tretinoin). I would also recommend using PRP during your microneedling treatment.Hope this helps!