The citation you provide is from Kent State in 1995. The current IOM guidelines is that adult males get at least 56 grams of protein a day which is consistent with the figure that @mrmikelawson gave. Given that protein is an essential nutrient (you cannot synthesize it from fat or carbs) it is probably a good idea to get at least 50g/day. If you ate a block of tofu a day you could get your protein requirement. But there are other sources, @roodgirl gives a bunch of ideas. As a vegetarian my daughter has struggled with protein as she doesn’t really like tofu and many of the other options are highly processed.
I think a harder challenge for vegan/vegetarians is getting healthy sources of fat as @still_young_at_heart mentioned. Humans actually need fat and important vitamins (A, D, E and K) as well as other nutrients can only be absorbed when you eat dietary fat. My daughter also supplements with B12 which is only found in animal food sources.
I actually think that someone with T1 can do better than a T2 on a vegan/vegetarian diet since they are likely insulin sensitive and can cover the increased carbs. Someone with T2 is insulin resistant and intolerant of carbs and generally has a harder time.