Attack with Holy Water as improvised weapon

Using holy water as intended takes an action and makes a ranged (thus DEX) attack.

A martially inclined character might prefer to forgo this 2d6 damage and instead use holy water as an improvised weapon as specified in PHB p.147 to deal 1d4+STR damage as a melee weapon attack. It is up to DM to determine the type of this damage and it might as well be bludgeoning not radiant. *

Using holy water in such a way might be advantageous, because you can use it with numerous class features such as Extra Attack, Divine Smite and Rage, as a bonus action with e.g. War Magic and improve it with feats such as Tavern Brawler.

Catapult

Similarily you could use Catapult spell to deal 3d8 bludgeoning damage by launching holy water

*DM could reasonably rule that bludgeoning someone with a flask of holy water in such a way would be attacking with a flask, not the holy water itself, thus not being applicable to negating vampire regeneration. An enterprising adventurer in response could freeze holy water i.e. with Shape water and use it directly as a bludgeoning implement. In such a case even if the holy ice dealt bludgeoning damage (which is perfectly reasonable) it would still negate vampire regeneration.

There are 2 cases where this distinction may be necessary:

• If you have holy water blessed by e.g. a god of fire that does fire damage, or cold from a god of winter, etc.
• If the vampire manages to get radiant damage immunity through some means, holy water will still work, if it's mixed with some acid, and now does mixed radiant and acid damage(even 1 acid damage is enough) while still being holy water.

The text is redundant. This is most likely due to the developers making changes to exactly how holy water functions during the extensive development process.

In D&D 3.5, holy water hurts undead "almost as if it were acid" and deals "2d4 points of damage" (with no type specified), and if they had similar language in 5th edition's pre-release versions, the text would make sense

That said, there's no harm in mentioning holy water in specific, so it's possible they elected to leave the redundancy in place on purpose, as an aid to players. If a player does not remember that holy water deals radiant damage in specific, they might fail to make the connection to the special effect on vampires, so the extra call-out could help remind people that "Hey, this right here is why holy water works especially well on Dracula."

It is pointless at the moment

As it currently stands, the specification is indeed pointless, unless your DM feels like ruling that a flask of Holy Water can be used as a bludgeoning improvised weapon to deal 1d4 without breaking, and this will somehow stop the vampire from regenerating, which seems extremely unlikely like it was the intended meaning.

As such, this may very well be an oversight, or some sort of future-proofing in case they ever want to make Holy Water that deals a different type of damage, or some kind of feature that changes the damage of Holy Water.

Are there vampires that are not undead?

If there were, they'd have to be fiends or they wouldn't take any significant damage from Holy Water to begin with. And if they're fiends, the mention of radiant damage would still not be needed because they'd be taking radiant damage. If a vampire could somehow become a different type that does not take the Radiant damage (for example by taking classes in Druid) it would take 1d4 damage of a type determined by your DM, and at that point it is up to them if this will still stop the regeneration.

But what if you absolutely want to make it be a useful plot point? Well, there is a way to make Holy Water specifically useful against a vampire, and that would be to give it a custom magical item of sorts, which protects them from radiant damage but not from Holy Water, for example:

Disclaimer: This is not a real item, but a similar item could exist in the future:

Sunshield Necklace, very rare magic item

Requires attunement by a creature with sunlight sensitivity

While wearing this necklace, the wearer is shielded from the harmful effects of the sun and radiant damage. Sunlight sensitivity no longer grants disadvantage on perception checks or attack rolls, and if the wearer would take damage from standing in sunlight, they are no longer harmed by the sun. In addition, when the wearer takes Radiant damage, they can use their reaction to turn the damage into Necrotic damage.

If a vampire was wearing such an item, being hit by Holy Water would not deal Radiant damage, but it would still be damage from Holy Water, so it would still lose its regeneration, which would not happen if it took Radiant damage from a different source.

As such, this may be a useful feature of the vampire in some future work.