What is the difference between acetic acid and volatile acidity?

Acetic acid is the main (but not only) component of a "true" volatile acidity reading.

A true volatile acidity (VA) reading measures ALL gaseous acids found within a wine and is traditionally measured using a steam distillation method as outlined by the OIV.  

VA acids include mainly acetic acid (≥ 90%), but also to a lesser extent other acids such as butyric, formic and propionic acids.

Important: due to the high level of acetic acid in a VA reading, it is not uncommon for VA to be expressed as only the acetic acid component (usually via enzymatic or NIR techniques).  This can lead to the interchangeable use of terms acetic acid and volatile acidity when measuring for acetic acid.  

Sentia measures acetic acid.  Be careful to only test like with like when doing comparative studies.