I have used Vanguards lately in endgame bounties just to see if I can get them to work without dying too much.
A thing I noticed is if the enemy fleet has anything with High Intensity Laser and points it at Vanguards, they drop like flies. HIL is too effective against all of the armor in the world. If the fleet does not have such a can opener, Vanguard sometimes dies a bit more often than other frigates, sometimes not.
If Vanguard was not so expensive relative to other frigates, it might be okay as it is, but it is too expensive. Centurion is similar to Vanguard and cheaper than it, and it does much what Vanguard can do and live longer for less cost. Vanguard needs to be cheaper.
Vanguard is Centurion Mk II. Even Centurion began flawed, with high cost and lackluster performance, although early Centurion's main problem was terrible firing arcs that effectively gave it firepower no better than a (Mercury) shuttle.