Pure skin dehydration can be easily fixed by moisturizers, but most of the cases skin looks dedydrated because it's inflammed, got to check out what you are allergic too, what is causing the inflamation, fungal or demodex or anything else. I had seboherric dermatitis before, skin was oily and dehydrated and lines, big pores, whiteheads, it's just very messy, i first though it's demodex but in fact fungal infection in pores with no visible sign of being infected by fungal (red spots). just go to the a dermatoloist who can do a KOH scrapping check for you. You will easily find that out.
if the dehydration is caused by vitamins difficiency, you should be able to see the dehydration happen to many parts of the skin. but if it only happens on face, you should expect there's certain infection or dermatitis going on.