In short said: In small intestine, mostly in oxidised form by active transport.
Up to ca 500 mg, it is absorbed through a Sodium dependent active transport (SVCT1= Sodium-vit C transporter, it is a cotransporter, I am sure
) process in enterocytes. In case of more than 500 mg, Intake occurs by passive diffusion. In oxidized form (dehydroascorbic acid) it is transported easily into enterocytes through glucose transporters (I guess it is also a cotransporter
). Dehydroascorbic acid is then reduced back to ascorbic acid by glutathione in the enterocytes. And then it enters the blood vessel from basolateral side.