vVO2 max is the velocity at which you achieve VO2 max, usually very close to 3k pace. vVO2 max can be improved by improving running economy or VO2max. VO2max can be improved by increasing mileage until you reach aprox 80mpw, after that phys studies show that increased mileage doesn't help much. You can also increase it by doing intervals where you get your body to VO2 max. usually 3k-5k pace intervals with rest intervals equal to or shorter than the previous pace interval. Although, if you do short enough rest and enough intervals 10k pace could work too or mile pace w/ longer rest. The longer you can get your body to spend at VO2 max the better, 3k-5k pace gets you there quickly and allows a good number of intervals at that intensity (see Daniels, Horwill, Coe, etc.).