It takes quite a while to get a deficiency, and also to reverse one. If you’re iron deficient it’s not going to give you an energy boost from eating some meat.
You can be vegan on beer and chips, and vegetarian on cheese and eggs alone, but it wouldn’t be a very healthy diet.
Nutritionfacts.org has something they call the daily dozen, which focuses on what you should try to include in your diet.
If you strive to follow that you are going to get all your macro and micro nutrients covered, except for B12. Everyone on a vegetarian, vegan or plant based diet should supplement with B12.
The page is abundant with science based information on what is the healthiest diet, and I’d encourage everyone to spend some time on watching a few videos on any topic that might be of interest.