I'm quite sure Jerome is correct. Dodge had nothing else in the parts bin at the time that would have worked (i.e. - no rear wheel drive cars), so the Dakota hubs were used to save on development costs.
Plenty of cars with as much or more torque than Vipers never have issues with "only" 5 lugs. The # of lugs are only part of the equation, along with stud dia., material, and length, as well as the space between them in terms of having sufficient material for wheel integrity where it actually bolts to the hub.