Seems to be.
So, the first thing that came to mind was: maybe the vampire stops being an Undead when it is in Bat form? But no, it doesn't.
Its statistics, other than its size and speed, are unchanged.
Notably, "Type" is one of the monster statistics, so even in Bat form it remains an Undead.
Are there vampires that are not undead?
Again: not as far as I am aware. All vampires in Curse of Strahd are undead as far as I remember and I don't know of any Vampire that is not undead in published adventures.
I also don't know of any RAW way to change the damage type of Holy Water, unless by some DM fiat on the rules about Damage Improvisation.
I also don't know of any way a Vampire could change its type while remaining a Vampire.
So, yes, within published material, it seems to be a redundant text. However, it may be applicable under some house-rules or under content published in the future.
It may be intentional - to make clear that Holy Water is really, really effective against Vampires - or it may be an oversight from the writers and editors. Either way, no harm done, I believe.
As discussed in the comments, there is an edge case where this might (very weak might - it still is up to DM interpretation) show up. A Druid player character turned into a Vampire. From the Monster Manual:
The game statistics of a player character transformed into a vampire spawn and then a vampire don't change, except that the character's Strength, Dexterity, and Constitution scores become 18 if they aren't higher. In addition, the character gains the vampire's damage resistances, darkvision, traits, and actions.
So, a Druid turned into Vampire would still have Wild Shape, from my reading, and would have the Regeneration feature from the vampires.
Then, it could use Wild Shape, which changes the creature type, becoming a Beast (or something else like Elementals for Moon Circle). Wild Shape then states
You retain the benefit of any features from your class, race, or other source and can use them if the new form is physically capable of doing so.
So, now it is up to the DM: is the new form physically capable of the improved regeneration provided by the vampirism? If (big if) the DM accepts that it does, then you now got a Beast with Vampire's Regeneration. Holy Water would not deal radiant damage, but would deal some damage (improvised damage), and would still stop the regeneration. Is this intended? I highly doubt.