Vg is the speed that will give you the best L/D aerodynamically in a given aircraft configuration and does not include environmental factors.

L/D is simply the aircrafts (coefficient of lift / coefficient of drag), and is only airfoil related. The best glide angle is not necessarily best L/D because now we are speaking in terms of distance over the ground vs altitude. The best glide speed & angle is all inclusive and factors in wind, lift/sink, L/D, & weight. The overall best glide speed is not necessarily Vg.

Actually, I should correct myself... L/D can be purely dealing with airfoils but in the terms we are speaking about it it is really calculated from flight test data... The flight test accounts for the aircrafts overall L/D which also includes lift and drag produced by the fuselage, etc. The test involves doing sawtooth climbs/descents and power off descents...

Some interesting reading...

Talks about glide angle regarding speed & lift/sink. It doesn't talk about wind, but to factor for wind all you do is move the origin of the line in the x direction.
Glider Performance Airspeeds

Detailed paper that debunks and confirms 1/2 wind speed rule of thumb. Talks about about best range speed but can apply to best glide as well. The author, David Rogers, also uses his E33A Bonanza to address some other interesting things.
(Enter the site and find the article "Wind Effects on Maximum Range")
Prof Rogers

Sawtooth climb flight test...
http://www.dfrc.nasa.gov/Education/O...f/sawtooth.pdf