I know that Vari*Lite is known for advancing model numbers without changing the actual fixture much. For example, I have some VL500's which are technically the same as VL550's. The only difference was that more fixtures were added to the series. Same with the VL1000/VL1100.
The VL2415 is the "beam projector" version of the WashFixture 2416. Basically the same fixture but in the 2415 there is no ZoomLenssystem. These both are the 1200 watt models.. 2413/2414 were the same units with 575w lamps 2411/2412 were with 1000w Incad lamp model.