TeamLapen/Vampirism#952 (comment)

When e.g. trying to fly as a bat while having the levitation belt equipped you get a levitation or slow falling effect.

The belt adds the potion effect depending on your ground contact and the shift key.

Of course players can just unequipped the belt to avoid such issue, but it would be cool if the belt could check if the player is already able to fly or able to fly.
Vampirism sets PlayerEntity#abilities#allowFlying = true to allow creative fly and while in air PlayerEntity#abilities#isFlying == true.

Maybe you can add a check for one of this abilities to the belt.