heres some facts
Vl hub takes a different fan bolt pattern. VL hub will fit SR20 fan and the new GKtech aftermarket fans but not any rb fan (other then the rb30 the VL has) just about every aftermarket supplier sells these hubs
R31 fan hub takes ANY rb fan, is shorter then RB20 and doesnt have external spring on it. very hard to get one of these aftermarket due to everyone sending out a VL hub. I just kept going to wreckers and grabbing 5 r31 hubs and then using the best one.
you can run two combos for best cooling... a r31 hub with an 11 blade latemodel RB25 fan or a VL hub with the GKtech fan. both will flow large amounts of air.