Get some vitamin d supplements and start going in sunlight more frequently, take calcium rich healthy food

Hello I dont think the hand stiffness is related to vitamin d but its related to how u sleep at night . Sleeping with arm or hand under the head is ok and stiffness in morning due to thatbis quiet rational , although since u have vitamin d low u can definately start taking tablets for the same and bring in a habit of stretching ur arms in the morning. If you have tingling and numbness only then a radiological investigation is warrented

It may be due to sleeping position causing some nerve to compression  May be due to carpal tunnel syndrome