The armour deck in Vanguard was put in at a higher level in response to the growing air threat at the time these ships (KGVs , Lions and Vanguard) were designed .
You have to remember that this threat was as understood at the back end of the 1930s , so you are looking at the Dive bombers of the START of WW2 , NOT the potent machines in use at the wars end .
The biplane Curtiss Helldiver , Blackburn Skua , Henschel 123 , Arado 81 , early JU87, the early Aichi Val , the early Nakajima Kate , Douglas Devastator , Vought Vindicator and similar aircraft , and Amour piercing bombs averaging 500 pounds or 1000 pounds at the worst .
Given the time lag between fixing the armour parameters of any battleship design , and actually sailing it out of harbour fit to fight , as being five years or so , and that in this period military aviation advanced from the last biplanes/first monoplanes stage to that of Jet propulsion , near trans-sonic operation and near-space travel , It is difficult to criticise the ship designers for failing to foresee an unprecedented and nearly Sci-fi rate of progress and increase in threat from the air , forced on in the wartime technological race in Aviation .
Looked at in the light of the envisaged airborne attackers of the time , Vanguard's defensive scheme does make sense , and you have to give the British design teams the credit for seeing that an opponent battleship's projectiles were no longer the only thing to take into account in any protective layout .
" Relax ! No-one else is going to be fool enough to be sailing about in this fog ."