I asked a gas company tech about this and he told me the newer gas ranges don’t need venting. I thought about it and concluded it has something to do with pilotless ignition.
I am a licensed plumber!
Do not vent anything into that water heater vent!
Gas ranges do not need a vent.
Sometime the range hood is vented through the roof to take odors and such to the exterior!
To, successfully tie two vents together you would need to increase the size to the proper diameter.
Plus vented gasses have to have heat to work so the range would never generate enough heat to vent anything out in conjunction w/ the WH vent!