I’ve lived that lifestyle for 23+ years. I’m healthy and energetic, and people usually guess my age as about 10–15 years younger than it really is.
I had an ethical motivation for going veggie, so the decision wasn’t difficult for me. When I was ready to make the change, I dove in and never looked back (no, I’ve never cheated or regressed, not even once).
For protein, just about all foods contain amino acids. If you eat a well-balanced diet, you probably won’t need to worry about getting enough protein. You don’t need to eat every amino acid at every meal; as long as you eat healthfully, you’ll get a full spectrum that combines easily. Also, if you remain concerned about your protein intake, legumes—peanuts, dried or canned beans, soy products, peas—are protein powerhouses.
Thanks to long-term and sweeping “education” campaigns by the meat and dairy industries, most Americans consume way too much protein, and usually at the expense of other nutrients.
For vitamins, you can’t obtain B12 from plant sources. You do produce B12 in your intestines, but there’s some ongoing disagreement about whether the B12 is excreted rather than absorbed (B12 is produced below the ileum, where the vitamin’s believed to be absorbed). Given the lack of a reliable conclusion, I take a B12 supplement every day…just in case.