What type of vapor barrier and concrete should I use for bare earth in basement?
A vapor barries usually cosist of 4-6 mil plastic, followed by 1" down to pea gravel stone by up to 4" thick. This will help with weed, mold control and drainage. Local codes may influence this, so I would check with your local building inspectors office. 

The concrete utilized should be a 3500 psi strength, since it's in a basement, and your not driving on it. If you decide to use concrete (over plasic and stone), it should be 4" thick ( 3 1/2" nominally). Good luck!