Can a vegetarian diet be sustained without supplements?
Lacto-ovo vegetarian. No problem. Vegans need to supplement, though not much really. B12 and perhaps some DHA from an algae supplement. But vegans can't afford to be sloppy with their diet. Most aren't sloppy, and follow a nutrient dense whole foods diet, but there are some "oreos are vegan!" type vegans out there. Omnivores can be a bit sloppy and still cover what they need although any sloppy diet will have consequences beyond the nutrient issue.
Personally, I think everyone should supplement. There's a wide range from optimal to deficient. Most nutrient calculations are based upon what you need to avoid overt deficiency. But I just don't want to avoid rickets and scurvy, I want optimal health and performance.
Let's call it nutritional headroom. I want to know that even under the most stressful conditions I'm not bumping into my nutritional ceiling.