[@Arkitekt]

Well the whole point of thieves' cant is for it to require a sort of cypher which you learn, so that not everyone can understand. For example in this instance the word [me] would be something like "a tug on the ear", most people would think it has something to do with listening, at least that's what they'd think of first. 

So it's less pointing and shaking heads as a code that is not understandable by anyone without the cheat sheet (basically knowing the language). 

I will say though, that Lautrec would attempt the pointing and things like that if he saw that Y'Vanna doesn't understand, as he's trying to get a point across (introduce yourself first), and not a message that is less likely to be understood by prying eyes. So if you want, it is completely fine for her to understand what he wants from her.

EDIT: I do like your suggestion of how easy or difficult it is to understand though.

Easy could be something that is universally understood by thieves who learned the cant, and difficult etc. could be something that is unique to the group that was established within themselves.