How are the Van der Waals constants a and b related to each other?
Yes, they are certainly correlated as shown in the following plot based on data from the Wikipedia Van der Waals constants data page  



The correlation should not be entirely surprising: the covolume b is a measure of the size of the gas molecules, whereas a is a measure of the strength of intermolecular interactions which is expected to depend through polarizability and dispersion interactions on the size of the molecules (molecular volume).

For a given gas parameters a and b tend to increase together. The shown correlation is empirical but since both a and b depend on molecular size and apparently in a very similar functional way, the result is a linear correlation. 

Following the hint from Karl, here is a version of the plot with log(a) along the abscissa, and with superimposed labels.