>Cattle are fed the byproducts of human agriculture that humans refuse to eat. Anything else cattle are fed (scrubland plants, grass, etc.) is extremely low-impact.
And there is no reason those byproducts can't be repurposed for other uses. You say this like they only eat byproducts, yet 70% of the soy production in the US goes to feeding cattle.
>The vast majority of vegans I know are clearly malnourished.
How many vegans do you know exactly?
>visible indicators of extremely good health
Visible indicators are a dubious way of measuring health. I also get the sense that you are comparing vegans who aren't interested in personal health to omnivores/carnivores who are. In other words, the vast majority of vegans who aren't interested in their health are ... unhealthy. The vast majority of people I know who eat animal products have high blood pressure and are at least pre-diabetic among other things.
>whereas it's very easy to do so from animal products.
We wouldn't be supplementing dairy with vitamins if this were actually the case. If you're talking about complete proteins then soy, buckwheat, hemp, chia seed, and spirulina fit the bill. If grown locally, quinoa. Already common combinations that make complete proteins are beans and rice, barley and lentils. This isn't anymore difficult than cooking a steak.
If you're talking about vitamins, then I hate to break it to you but you have to supplement even on an animal-based diet.