Despite healthy lifestyle, no diabetes, normal blood test results, i have dry skin between fingers, around eyes, the back of tongue is always white, it started a year ago and it doesn't go away. i drink enough water. could it be a vitamin deficiency?

These things do not sound abnormal. But see a doctor if you remain concerned.

Dry skin, especially in the hands and between fingers can be due to much hand washing, without adequate moisturizer use. But it can also be from contact allergy//hypersensitivity with some irritants in some gloves, chlorinated water in pools or even occupational hazard from work like a mechanic or handyman. Yes Vitamin D and some vitamin B deficiencies can also be implicated. See Dermatologist