I looked at some of the other expressions using "What a.." and some of them use 'qué' while others use 'vaya'. So would it be "Qué mentira" or "Vaya mentira"? What is the difference? Thanks in advance.
I think we tend to use "qué" with an adjective, as in "¡qué mentiroso!" (What a liar!), and "vaya" with nouns, as in "¡vaya mentira!" (What a lie!).
You can also say "¡vaya pedazo de mentira!" (What a big lie!) or "¡menuda mentira!" (menuda = pequeña, but here it's ironic, it means big)
I'm sure there are lots of other ways to convey the same. This is what comes to my mind.
Here, in the South of Spain, we say: ¡Vaya mentira! or ¡Qué mentiroso! as child, or in a colloquial contect, but it it not elegant, nor educated.
But we are tought to say: ¡Eso no es verdad!= ¡Eso no es cierto!=¡No es verdad!= ¡No es cierto"
It is a similar thing in English with: "I want, never gets".