I dont consider Vypers an option, an armor 10 unit with 1 BS3 Shot is a joke and not competitive although it can be used to some effect.
I don't get this, War walkers are just as fragile (maybe even more so, because you can't zoom away out of
LOS/for a cover save) but they take a valuable
HS slot. Where the Viper takes a otherwise pretty useless
FA slot. Which is huge, if you ask me.
A dual Shuriken Cannon Viper cost as much as a dual Scatter Laser Warwalker. The
ww has more shots and range, but the latter doesn't hurt he Viper to much because of its greater mobility.