Can people north of 35deg latitude get enough Vitamin D via the sun in winter?

NO.   Human bodies possess a factory that makes vitamin D; it is called the skin. Winter clothing shuts down 95% of that factory (face and hands is only 5% of skin surface). Beyond that, a rule of thumb is that you need the sun to be at least 45 degrees above the horizon (a UV index of at least 3) for it to provide the skin of a white person enough ultraviolet light to make a useful amount of vitamin D. 

Bear in mind that skin pigmentation acts like a sunblock, so that a very dark-skin person has the equivalent of SPF 6 sunscreen naturally built into their skin, and needs six times as much ultraviolet light (a combination of sun intensity and/or exposure time) to make the same amount of vitamin D sdas a White person. All populations outside the tropics exhibit declining levels of vitamin D nutrition during winter months. Fortunately, vitamin D disappears slowly out of the body, so our summer gain in vitamin D helps to prevent severe deficiency during winter. The prudent thing is to keep supplementing with vitamin D all year.