Hi h & k,
WELCOME TO THE FORUMS ! ! ! Thanks for joining us.
I guess we'd consider as irregular plurals, things like deer, deer; man, men; child, children; hoof, hooves.
"Vocabulary" can be either countable or uncountable.
His English vocabulary comprises only a hundred words, but he has extensive vocabularies in French and German.
Improving vocabulary is a major goal of the new reading program.
Best wishes, - A.