Hi guys, I´m mexican and the law that bans e-cigs it´s one that also bans chocolate cigarretes... According to this law, it is prohibited to import or comercialize anything that looks or resembles a cigarrete, it is not illegal to use it.
You may have a problem with customs officers at the port of entry, if they want to squeeze some bucks from you. Usually they don´t make a fuss unless you have lots and lots of stuff, since that would indicate intent to sell.
You´ll find vape kiosks and stores all over, and they are not allowed by law, but permitted. The only authority that can legally give you a hassle in respect to e-cigs are the customs officers (aduanas) or Cofepris officials, wich are in charge of health concerns (and I have never ever seen a cofepris official in my life).
So, just to be safe, if asked, call it a vaporizer and don´t flaunt it at the customs officer. After you pass the checkpoint, enjoy!