What class of histone deacetylase (HDAC) inhibitor is sodium valproate (VPA)?
This is explained at the WP page for HDAC. There are five classes:

hydroxamic acids (or hydroxamates), such as trichostatin A
cyclic tetrapeptides (such as trapoxin B), and the depsipeptides
benzamides
electrophilic ketones
the aliphatic acid compounds such as phenylbutyrate and valproic acid

and valproate is in the final class, aliphatic acids.