Vanguards are using support items because they lost their role of Tank carry.
People think Tank is one, like Juggernaut + Wardens + Vanguards are one. That's totally wrong.
Juggernauts are the Defensive Fighters in Backline that shouldn't start the fight and wait till enemy team dive them or target someone. They scale well with damage Hp and they got a lot of sustain helps them Shutting down any target who is targeting their team
Wardens are meant to be Protectors, u can go them full tank support and do your job by Peeling using your Cc to help and protect, staying with your ally adc and help him at all costs to prevent him from dying faster.
Vanguards are the carry tanks of late game, They are meant to Start the fights, they are meant to shut down any squishy target on their way, with all these nerfs and changes, they are no longer doing the job they can't buy the Vanguard items other than only Abysal mask which helps giving them brief sustain + extra magic damage.
Adcs are literally destroying on vanguards because the itemization problems since they got way weaker and they deal way less damage, That's why we see Vanguards using Support items like Knight vow or locket etc
Riot you gotta fix that problem Asap, just take 1 minute thinking about vanguards and you'll figure out how bad they are compared to the rest they literally got no role atm other than being the 2nd Wardens who just peel ur team