The question is, why do you say you are a vegan? The answers are personally so varied as to fill a tome.
Most vegans are conscientious, making choices that reflect their passion to "do no harm" to another, in order to live this life. I feel this way. I don't want another to suffer in order for me to live a good and valuable life.
But there are some who do it for vanity's sake. A friend I have lost weight doing Beachbody, which I am a coach for as well, and when they offered their Ultimate Reset plan, we both jumped on it. The difference is, that I realized then, even though I'd been vegan for 5 months already, how truly great cooking vegan is when you followed a recipe. Being vegan was no longer a thing I do to help animals, it became a true joy. (If I don't follow a recipe, let me tell you, my food has a 50/50 chance of tasting very bland.)
Then comes the question, so why is it a bad thing if becoming vegan is done for vanity? Well, the answer is, it really is not a bad thing. If the tool needed for someone to become vegan is purely for vanity, then so be it. The fewer people who eat animals the better.
The problem comes when the next time the tide changes, she may go back to meat eating because vanity is fluctuating. Simply put, if you don't believe in something on an ethical or moral level, your dedication to it is subject to change.
People are wind driven, that much we all know. When the great diet wind shifts, Paleo-eating comes to mind, everyone jumps on that sail boat because for the moment a few of the higher profile people could eat all the meat they want, blame all their ills on grains and fruits, and live, (forgive me, pigs) high on the hog.
Excuses abound to blame one grain or fruit, or berry, so people continue to eat meat, thanks to the meat industry. I googled "meat based diets" and came up with one after the other explanation for denegrating plant based diets, going back as far as the first human! (http://www.google.com/search?client=safari&rls=en&q=meat+based+diets&ie=UTF-8&oe=UTF-8) Look at the one link that says the first humans didn't do very well until we started to eat meat. Who came up with that?
The way I see this explanation, it was a matter of ease. It is easier to kill and consume an animal than it is to reliably find the best plants day after nomadic day. Animals were everywhere, but someone, at sometime, must have said, "who remembers where that grove of blueberry bushes was? Anyone? Ok, there's movement, lets kill it. I'm tired..." Granted, this is a very simplistic reason for what must have been a very hard ordeal.
But that was then. They are not living now, when we have evolved, when one out of three Americans is fat, when diabetes is an epidemic, and when factory farming tortures, pollutes, and hoards grain stock that could feed the millions of hungry people in the world, when instead they are lopsidedly feeding chickens, cows, pigs, and horses, for human consumption.
Obviously, we are far beyond the paleo era. If factory farming instead grew more of the super foods, grains, trees, and grasses, fruits and vegetables, the world would be well fed; the idiots with torturous tendencies might not be so prevalent, and of course, this planet would be a better place.
So, if vegan because your weight is an issue, then all well and good. Every vegan and every excuse to be vegan is still a vegan. My hope is that if for vanity, then strive to keep that way. My belief, is that even the vain, even the topical reasons will eventually find roots in the sensible. It is the non-vegans who will challenge a person to drop off of it. But I say, just do it. Go vegan, then by God, stay vegan. Any reason to become one, can be led to grow into a permanent, seeded, and fruiting tree. It just needs some water, and some light.