The most efficient way of getting vitamin D would be from sunshine.
"If you live north of Atlanta (ahem, Boston), it’s impossible to produce vitamin D from the sun in the winter, because the sun ray’s never get high enough in the sky for the ultraviolet B rays (UVB) to penetrate the atmosphere. This can be a problem for most of us that aren’t lucky enough to have winter homes near the Equator."
source: http://www.bostonmagazine.com/health/blog/2013/01/11/vitamin...
And natural synthesis reaches an equilibrium after about 15 minutes of exposure.
So for many, the most efficient way is swallowing a 10000 IU capsule 1 to 7 times per week.