Here's why Gragas shouldn't be considered a Vanguard, but a Diver instead

http://www.mobafire.com/images/champion/skins/landscape/gragas-scuba.jpg

He's clearly diving in this splash art.

And I've never seen Gragas guarding a van before

https://i.gyazo.com/c2855d4cc38b913d373165bcbb8a3401.png

except maybe this time.