Since the HA is made to kill devils and demons does this negate the resistance that a demon/devil would have to normal magic weapons?

At our table GM chose to "bless" a sword. Our Cleric began their travels after their vilage got wiped. The urge for retribution gained them a level of paladin and set them up for the oath of vengence. Their God alowed it, seeing how much sorow it caused the Cleric to lose so many people they knew. And the God in question had its own gripes about the folowers lost. Much later, Party took a detour to asault the fort where we had intel the vampire count responsible for the slaughter was. A session long batle later, Cleric has a vison of their God manifesting, GM had a long speech ready, at the end of which, GM described how God touched the blade Cleric was holding and a pristine aura erupting from the blade, soothing everyone nearby, as the formerly crass metal now sparkled with intricate carvings and pristine glisten. Anouncing that Cleric/Paladin's sword has become a Holy Avenger

Give it to them early but make it's abilities diminished/limited. (i.e. they find the weapon but it can only be identified (at the time) as a +1 instead of the listed +3, and it only does 1d10 radiant but only against a couple types of creatures but no aura effect. AND you can make it unpredictable. Have the player roll percentile dice to see if the radiant ability even works. And then, based on what you as the DM decide. Once you "allow" the character to attune to it, keep the stats as is but no longer unpredictable and make the weapon a vestige of divergence.  As the character levels, improves, does awesome stuff in game, ect. Add more of the features to it until it has all the abilities of the holy avenger. So essentially they get it at like level 5 but only as a +1 weapon.  And then in world you can explain something like "the character knows that there is something different about this weapon and but they can't quite place it.  The energy of the weapon feels familiar to them and they just "know" this weapon can do more." Or something like that. And based on what you set as improvement requirements, add a +1 here, and other 1d10 Radiant, add the aura but just for the character at first,.and then expand the aura up to the 10 foot radius as another improvement. Then, by like level 13 or 15, have them fully attune to the Holy Avenger with all abilities.  Basically turning their own weapon into the Holy Avenger. 

I do this method as a DM too. I find that the players are a bit more committed to "the role" of their characters and focused on what's going on in the campaign vs walking around and hacking everything for XP. If they know early that they found a "special weapon" but don't know what it is or what all it can do but also know it can do ALOT more, they will.work to unlock all of it's features. So far, each time I've done the Vestige of Divergence "style" weapons, my players have loved it.