It's probably doable, but it's a lot more work than just eating meat or fish.
When i was younger i went for a full vegan diet for about two years. I went from 160lbs to 120lbs and gained about 10 inches in height. I definitely did it wrong. Mixed diet with some meat in it you can just half-ass it and get roughly everything you need without trying.
Cutting either meat or vegs and you really need to supplement what you're missing one way or another.
Plasma donation with vegan diet would require a lot of work. You'd be eating all the time. Meat is really condensed protein and it's not the only thing you get from meat. Fairly sure even pescatarian diet means eating more. Mixing everything is just least hassle. Fish oils have their place and you roughly hit the spot eating fish once or twice a week.
Depending where you live also changes things. There was some old thread where Hongkonger wondered how people from Northern-Europe dealt with vitamin D. It's was just the traditional diet with everything you can make from milk, drinking milk with meals and fatty fish like salmon. I think parts of US close to that besides Alaska.