Here is how the vB reputation 'dots' work:

One green dot () is given for every started 100 reputation points.
From 500 points on, one bright green dot () is given for every started 200 reputation points.
One red dot () is given for every started 100 negative reputation points.
From 500 negative points on, one bright red dot () is given for every started 200 negative reputation points
If a user has 0 reputation points, a gray dot () is displayed.
If a user has chosen to turn off reputation in his UserCP, a black dot () is displayed.