We have to be careful not to offset the health benefits of a vegan diet. There is a surprising amount of evidence suggesting that low-normal iron stores are beneficial and may reduce cancer incidence, mortality and perhaps increase longevity. Specifically, as strongest, I would point out the FeAst study and numerous recent Mendelian randomization studies on iron and longevity (e.g. Daghlas and Gill 2021). It is prudent to test ferritin to know whether you are too low or too high.
Vitamin D testing certainly could be useful, even though recent clinical trials testing vitamin D supplements are between somewhat to highly disappointing, but deficiency is presumably not strongly linked to veganism.
I think the consensus among nutritionists is that a well-planned vegan diet is among the healthiest possible diets. Almost everyone in the US would benefit from “going a bit more vegan”. Nevertheless, it is probably not optimal on certain axes.
It would seem that the best diet to improve long-term health is a flexible pescolacto-vegetarian diet supplementing certain carninutrients, e.g. creatine. So not vegan.
Tradeoffs are real and you have to optimize for one thing over another. For example, a standard (unsupplemented) vegan diet may not be optimal for mental and physical performance due to the higher risk of iron deficiency, low protein intake and lack of dietary creatine intake, among other things. A lot of those issues can be alleviated through careful planning. However, it may very well be that low-moderate intakes of protein and iron are one of the reasons why a vegan diet is healthy and you will have to weigh this against the potential performance aspect.
We have two types of evidence in favour of veganism:
We can extrapolate that veganism is healthy from first principles by studying individual foods and nutrients. For example, we know that saturated fat often found in animal foods increases LDL which is a validated marker of cardiovascular disease. Saturated fat intake will be low on a vegan diet. Another example is iron, we know that even unprocessed red meat is problematic because it is a rich source of iron and we know that many men consume too much iron (whereas some women consume too little). We know that iron intake and availability will be lower on a vegan diet and we have relatively strong evidence that elevated iron levels are harmful to health.
Or we can be lazy and just take a bird’s eye view and look at mortality data in observational studies. These studies clearly show that both vegetarian and vegan diets are associated with reduced mortality and superior health outcomes. Unfortunately this kind of study design is subject to different biases like residual confounding, e.g. it may be that vegans are healthier than average even after we controlled for obvious variables associated with health like income, cholesterol, body weight, etc. While this does not prove that veganism is healthy, it definitely shows that veganism is consistent with above-average health outcomes.