It depends on who you ask...

The above might be overstated a bit...but, not by much.

For instance, "Research supports a range in protein needs from 1.2 to 1.4 grams of protein per kilogram body weight for endurance athletes such as marathoners". That's 0.64 g / lb. For a 170 lb athlete, that works out to 108 grams of protein per day.

Or, "because deficiencies in total protein or in specific amino acids may occur, we suggest that athletes consume 1.8 to 2.0 g of protein/kg of bodyweight/day." At the upper end, that works out to 0.91 g / lb per day. For a 170 lb athlete, that would require protein intake of 155 grams per day.

These levels of protein, and the need to get a balanced mix of amino acids, make it pretty hard to achieve adequate protein intake on a typical vegetarian diet. Clearly, it's possible, but it requires a great deal of vigilance and dietary planning, when compared to non-vegetarian sources.