Vanity is just the selfish pursuit of happiness. All actions, remember, are part of the pursuit of happiness, even if they are poor choices, that's why they were made.
Also, yes, we are hard-wired to be vain. Our instincts guide us (this is all proven, but I can't sleep and I'm in no mood to find the book I read all this in) in most cases towards things that garner prestige for us. When faced with two options, the one which is on face a happier thing, and a harder thing which confers prestige, people almost always go for prestige but afterward when asked say that it was not a good choice.
Think of people who slave away at jobs chasing promotions, and in the end, what do they care? Most of these people realize they are a gerbil in a wheel after a while, but it doesn't mean they aren't pursuing happiness. They're just victims of their instincts.
It is however, not an evolutionary mistake. The reason we crave respect and prestige so much is that it is very very evolutionarily advantageous. Among social creatures like humans, prestige and respect are valuable things. Those who have it have more healthy children. Those who do not have prestige are often outcasts and do not do well. This is related to the evolution of mutual reciprocity, which eventually became the framework for almost all of our hardwired morality. It is the feeling that a free-rider has some bullshit coming his way. It is also the feeling that you owe something to someone who helped you.
Anyway, that's a tangent. But it is very interesting to me how our Darwinian-honed instincts so often drive us away from what will make us truly happy. You can think of it as the analog to the Devil in Christianity - that bastard that knows just how, in every situation, to make the dumb choice seem best, and you only realize this once the deed is done, every single time.
The struggle becomes working hard to learn as best you can what will truly make you happiest, and once you find it, fighting for it with every fiber. Working for all of humanity, in an egalitarian way, is what has shaped my life's plan. I think of it as my purpose. However I'm a normal guy, I give way to a lot of distractions. The battle is to only give way an appropriate amount. A balance must be struck. Surely leisure and being respected are important, but I have learned through research and experience that they are not the most important, not by a long shot.
As a side note, the craving we have for prestige basically single-handedly explains all examples of conspicuous consumption. People in general, when actually faced with a choice, choose a thing they earlier said they would not prefer just because it is conspicuous. For instance they choose to be given a new prestigious title over getting a free vacation. The vacation is pleasurable but incurs little prestige. Etc.
Also, yes, unhappiness makes you happy. The thing that is confusing here is that happiness has many meanings, all of which are being used in this conversation.
One simple meaning is bodily pleasure - that which we gain from leisure - food, sex, vacations.
Another meaning includes other more abstract things - seeing pretty landscapes, getting a lot of work done, helping out a friend in need. You could tentatively label this type of happiness "fulfillment." This is the good feeling I get when I think about the good things I have done, especially the things that were hard. I imagine it is similar to the feeling parents get when they contemplate the children they have raised.
In fact it is fairly easy to disprove your claim that my statement made no sense. Parents who raise children, when asked to report their happiness, show consistently lower levels of happiness than do those who do not raise children. Literally just about every single time.
So what gives? Why do people keep having kids? My suspicion is that these people, when asked about happiness, thought of simple pleasures. Your moment-to-moment mood. A lot of people think of happiness as just being really smiley and being in a good mood and getting what you want. That's only one, shallow kind. Most people know this but don't articulate it. We want more than that, we want all kind of strange, abstract things. And the wants often conflict. I might want to be an ascetic monk. But I might also really love cream puffs. Dilemma.
Anyway if they changed the poll to ask about fulfillment and satisfaction with direction in life I am confident that those with children would probably usually report more happiness in that respect. People, after all, continue to have kids. They do so because they believe it will make them happier to do so, in one way or another. In this particular example, the abstract thing that they get out of raising their kids is love. They give up a shit-ton of material things, their money, their health, their youth, their time, but they gain love, and many (most?) people find this to be worth it.
The same goes for a man who shuns happiness. Are there not examples of this the world over? I mentioned ascetic monks - what do you suppose their goals are if not exactly what I have described? And Buddhists who meditate with the goal of annihilating the self? Surely they are not pursuing personal happiness? I think they would deny it, but I believe they still are. They think that the only way they can really be happy is to exist outside of the self. Or something along those lines.
It is an interesting exercise to read about the belief system of any particular group, and read it with the pursuit of happiness in mind. It becomes obvious that when someone is trying to convince another person to do something or believe something (as a diplomacy player this will be obvious) that all of the truly convincing arguments revolve around what benefits will accrue to the person.
Follow the Buddha and find peace!
Worship Jesus and commune God!
Work hard and get to the top!
Join the army and serve your country!
All of these things will appeal to people who internally have a belief that "serving your country" or "communing with God" are good things, that is to say, that they are things that will make them happy.
In fact, all uses of the normative words like "should" "ought" "good" and "want" have the concept of the pursuit of happiness tied up in them. If I say "you should not smoke cigarettes," at the end of the day that "you should not" means that if you do, more unhappiness than happiness will result.
Thus the statement "why should I pursue happiness? What makes it a worthy aim?" becomes meaningless - a tautology. What you are asking is: "Why would pursuing happiness make me happy?"
Is the sky blue?