This VIN document filed with the NHTSA reveals that the Rivian delivery vans will simply be called the 700 Van and 500 Van (the 500 available in left and right hand drive). The number is the cargo capacity, with a 900 cubic foot version supposed to come at some point.

Van trims seem to include S for Service and Z for Amazon, the first possibly indicating that Rivian will eventually sell a version of the van to anyone interested in added them to their fleet.

We can also see that drivetrain options for the van will be 2x single-motor AWD, dual-motor FWD and single-motor FWD.

Check out the VIN decoder for yourself below. Rivian put the vans and the R1T on the same sheet so not all the information is for the van. You'll have to compare it to the R1T's separate VIN sheet on the left to see what is specific to the vans on the right.
Font Material property Parallel Pattern Number