It is most common to wear either a button-down dress shirt or a solid-colored T-shirt under a V-neck sweater. However, what a person chooses to wear underneath a V-neck sweater is a matter of personal choice more than anything else.
A button-down dress shirt is generally worn underneath a V-neck sweater for more formal occasions, and can be worn with or without a necktie. Casual occasions often call for a simple T-shirt instead, which should be coordinated with the color and print of the sweater itself. Regardless of what is worn underneath, a V-neck sweater can be worn with denim jeans or slacks.