The air conditioner does not create the Vitamin C. Ascorbic Acid is stored in some kind of material, comes to the surface in a defined way, and is carried with the air. The filters / dispensers have to be replaced when they are empty.
I could not find any references to the actual replacement parts. Some info about the Nissan system is here. Interestingly enough, they say Vitamin D and E are too heavy to carry in air. Vitamin C weighs 176,13 g·mol−1, D&E more than twice as much.
Siemens manages to make their information about the dispersed amount of Vitamin C quite unfathomable. They say it can reach up to 0.65 mg/l of Vitamin C in air. 20°C air holds about 20mg/l of water at 100% humidity, making about 10mg/l at normal household humidity of 50-ish %. So 6.5% of humidity is available as Vitamin C. Also, the unit delivers about 800 m^3 / hour. This does not quite add up: about half a kilo of Vitamin C per hour.
From what little the companies divulge, it seems to be possible to dispense amounts of Vitamin C about as heavy as the truth in these papers into the air, which then might get absorbed by a human body before being lost.