One-fourth cup may or may not be enough depending on the pup. What is the vet saying about weight?
From a scientific standpoint, I see no problem with a vegetarian or vegan diet. I'd much rather see somebody go ova-vegetarian if they wanted to go this way because egg is an amazing protein source for dogs. It's not hard to balance either (relatively). However, that doesn't mean that healthy dogs can't do well on vegan food. Dogs require amino acids and nitrogen, not "meat" or "poultry" or "fish".
I'm not sure that there is any scientific evidence anywhere showing that dogs have to have animal products to survive.
If we're talking about bioavailability of the protein source, then it's actually surprising that more people don't choose egg or red meat...
Ry loves Ellie's soy nuggets. If it wasn't for the fact that I don't want him on a hypoallergenic diet, he'd be a vegan dog too.
Over the time that we've had soy kibble in the house, I've become very fond of it - much fonder than I am of six month old animal based kibble.