Vampires and vampire spawn have regeneration. But..
if the vampire takes radiant damage or damage from holy water, this trait doesn’t function at the start of the vampire’s next turn. (MM 297, 298)
Holy water allows you to make a ranged attack. And…
if the target is a fiend or undead, it takes 2d6 radiant damage. (PHB 151)
What is the point of specifying that vampire regeneration is turned off by radiant damage or holy water if holy water does radiant damage to undead?
Are there vampires that are not undead?
While it may be possible to turn one spell damage type into another, I can’t find an example of being able to turn object damage from one type into another (that is, a way of making holy water do non-radiant damage but still being holy water).