I recently read that vitamin C reduces the effect of rogaine. Is there any truth in that?

Where did you hear that? There's nothing in the Electronic Medicines Compendium about any interactions between Regaine Extra Strength and vitamin C.

http://emc.medicines.org.uk/

4.5 Interaction with other medicinal products and other forms of interaction

Topical drugs, such as corticosteroids, tretinoin, dithranol or petrolatum which alter the stratum corneum barrier, could result in increased absorption of minoxidil if applied concurrently. Although it has not been demonstrated clinically, there exists the theoretical possibility of absorbed minoxidil potentiating orthostatic hypotension caused by peripheral vasodilators.

It turns out that I missunderstood the article I was reading. Thanks anyway