The official selling point of Vilebrequin's swim trunks is that they have a unique vent system that prevents unattractive air bubbles from forming when you get into the swimming pool or ocean.
The reasons the colorfully printed, traditionally cut designs are so popular along the Upper East Side - Palm Beach - Hamptons axis, however, are probably their comfortable similarity to other collect'em all ruling class prints like Lily Pulitzer dresses and Hermes ties. And their forgiving shape, which doesn't make it too obvious that your officebound shape needs a bit of forgiving itself.
Also, by buying dad-son Vilebrequins in matching sets, it makes it easier for investment banker dads to recognize their own offspring, who they otherwise would only see for about 20 waking minutes a week.
Vilebrequin Costumes de Bain on Madison, online, and as above, in Aix-en-Pce, $165-175 for men, $90 for boys [vilebrequin.com]
Fancy schmancy swim trunks for boys - $90
Average boys swim trunks from (pick a name) department store - $15-$30
Watching your sons laughter as he spends hours making "bubbles" in those "cheapy" swimming trunks - priceless.
That is all.
Forget the bubbles - do they make heated ones so it doesn't take me 1/2 hr to get past, uh, you know, THAT point when getting into the pool..?