VV will keep the output voltage the same, even if the resistance of the atomizer or cartomizer changes, and as batteries deplete.
VW will keep the total power (watts) the same, even if you change the resistance, and as batteries deplete.
Watts = Voltage x Voltage / Resistance
Both approaches can work well, IF the regulation (whether V or W) is good. VW is technically superior in that it will keep the heat (power, watts) the same as resistance changes, although at a practical level I don't know that it matters much. Realistically, at least the cartos/atties I use don't change all that much over any given span of time, other than for something having a sudden failure (atty pop).
Some lower end regulated devices don't work quite as well, have difficulty maintaining the set voltage/wattage, but well regulated devices will indeed maintain right up until you need to swap batteries.