Food will only remain in their system for a couple of hours, as compared with about 24 hours for omnivores like ourselves or days as in most herbivores. They are adapted to eat relatively small amounts of highly digestible, energy-dense food, with an optimal level of vitamins, minerals and micronutrients. Proteins derived from vegetables are much less easily digested, lack many of the vitamins, minerals and micronutrients that are necessary for complete absorbtion and must be consumed in much greater quantities to obtain sufficient energy for healthy functioning.
Humans, whose need for high-quality protein is far less than those of cats, have a difficult time remaining optimally nourished on a solely vegetable-derived diet, and must pay particular care with food types, quantities and combinations, as well as take a variety of supplements, to avoid becoming malnourished or suffering from various deficiencies. I find it highly unlikely that any vegan diet could supply all the nutrients a cat needs to thrive.
While I can appreciate the philosophical stance that can lead to a human's eschewing all animal-derived products, and recognise that they have a perfect right to opt for this for themselves, it can only be regarded as cruelty, and ultimately abusive, to force a cat to follow such practices. They need meat not just for the protein, but for all the other factors that are found in it, for its concentrated energy and for its speed of digestion. For a cat, plant-derived foods are simply not good enough.