Answer modified to discuss the entire Shuttle Reaction Control System not just the Vernier RCS
The shuttle RCS was divided into 6 VRCS jets and 38 Primary RCS jets. It was also divided into three pods - left aft, right aft, and forward. Each pod had its own helium pressurization, fuel, and oxidizer tanks. The aft RCS pods could be connected to each other to share their propellant supplies. There was also a 6000 lbf Orbital Maneuvering System (OMS) engine in each aft pod with its own tanks. The plumbing for the OMS engines could be interconnected with either or both aft RCS pods. There was no interconnect with the forward pod. The plumbing is both interesting and complicated - see the references for details.
The VRCS jets burned nitrogen tetroxide and monomethyl hydrazine at a mixture ratio of 1:6 and a chamber pressure of 110 psi. The thrust was 24 lbf. The expansion ratio varied from 20:1 to 50:1 (the jets were cut off ("scarfed") to fit the outer mold line of the vehicle). The maximum continuous on time was 125 seconds, the minimum was 80 milliseconds.
There weren't any up-, forward- or aft- firing verniers, only left, right, and down.
On these following diagrams, the verniers are always in manifold 5 so their names are X5Y where X=pod Y=direction.
The Primary jets burned the same fuel and produced 870 lbf at 152 psia chamber pressure and the same mixture ratio. Their expansion ratios varied from 22:1 to 30:1. They had the same time constraints but allowed for a contingency deorbit burn of 500 seconds for the aft jets.
All info from the 1982 Press Manual
You can get some awesome pictures of the aft of the shuttle including the jets from the inspection photos taken from ISS, like this one.
Besides the Press Manual, there is a lot of great information in the Shuttle Crew Operations Manual section 2.22 about the RCS.
For information about using the RCS to maneuver the vehicle, see this question: How fast can the space shuttle change its attitude in space?