I am vegetarian, not vegan. One reason is practical. A vegan diet requires more planning. But it is certainly possible to get all your dietary needs by adhering to a vegan diet.
If you are contemplating it for yourself, I would suggest that you inform yourself well about it. I don’t think you need necessarily a dietitian but I would get some software that analyzes your diet and makes recommendations.
I will also agree with Karin. And if you would like to try a great vegetarian meal plan (not completely vegan) here is a great site: http://www.thefresh20.com/
It has some awesome recipes and I have tried them personally. It’s worth taking a look and/or recommending it to someone you might know.
My daughters (who are both athletes, one is 22 the other 17) are very healthy vegans. They are conscious of what they eat, but I do believe they are meeting all their nutritional needs. It can be challenging, we mostly eat vegan (though my husband and I are not even vegetarians), but with good planning you can get your dietary needs. As the others suggested seeing a Registered Dietician can help.