The body needs methyl groups and it will take them anyway it can get them, 1 from methionine, 1 from sarcosine (methyl glycine), two from dimethylglycine, and three from betaine, carnitine, and choline.
The body needs sulfur and it will take it anyway it can get it – from cysteine and methionine, from sulfate, from plant sulfur compounds.
Without these other, non-methionine sources of methyl and sulfur groups, vegans would be short-lived people. How well these salvage pathways work in vegans is pretty much of an unknown. Doctors have no idea what is going on under their noses. Serious, sensitive diagnostic tests are needed to assess these salvage functions.