they don't actually do 26.6 dps, that's what they do when they get charged, which doesn't happen instantly at all.
vikings cost half the gas and 3/5 the minerals and have longer range, so depending on number of vikings vs voidrays you can prob expect a couple of the voidrays to already have died way before it even getting charged up
from TL wiki:
Quote:
On an equal supply basis, Vikings will be able to take out Void Rays