The bulk density in layers used in the VIC model can be compiled from data available through Penn State University at http://dbwww.essc.psu.edu/dbndx/tree/amer_n/us_48/data/soilprop/statsgo_geo.html
This reports bulk density for each of 11 individual layers, which cover a depth of 2.5 meters. The majority of data does not extend beyond 1.5 meters, which includes the first 9 of their 11 layers. The Penn State data is available in gridded form at a resolution of 30 arc-seconds.
The gridded bulk density data is imported into Arc/Info, where it is averaged for the appropriate individual layers and aggregated to the grid cell size. An example Arc/Info macro to perform this can be downloaded here.