A few things that we all notice about Vanilluxe:
-Vanilluxe specializes in Special attacks
-It can learn Weather Ball
-It can learn two different Weather moves that benefit it in different ways: Hail lets it wear down opponents, especially in combination with its Ice Body Ability, and Rain Dance halves the power of Fire-type attacks (which Vanilluxe is weak against) and doubles the power of Water-type moves (which Vanilluxe can learn, and Fire-types are weak against)
-It needs Hail active to activate its Ice Body Ability, can make good use of Water-type attacks to counter two of its Weaknesses (Fire and Rock)
Many of you have noticed one or more of these things, and built pretty tough movesets out of your findings.
However, I have looked at all of these, taken a "zoomed-out" look at all of these facts at once, and come up with what I believe is a "big picture" that takes Vanilluxe out of the shadow of other Special-attacking Ice-type Snow Cloak/Ice Body + Hail + Blizzard users and in a surprising and unique direction that I believe will unlock its true potential and make it a force of nature to be reckoned with in every Battle Format: Singles, Doubles, Triples, Multi, it can hypothetically adapt to any environment and makes a great teammate to many different Pokémon!
Ability: Ice Body
Held item: Leftovers
Ideal Nature: Timid or Modest
This Ultimate Vanilluxe can do what Castform was meant to do: change the weather in different ways depending on the situation, and power up and change the typing of Weather Ball to always deal Super-Effective damage.
Since this Weather Vanilluxe wins by adapting to each opponent and altering conditions to its advantage, I like to start off with Autotomize to make the somewhat-slow Vanilluxe into a somewhat-fast Vanilluxe that can out speed most unboosted opponents.
In a normal situation, it can use Ice Body + Hail + Weather Ball (Ice) to wear down opponents with a combination of Hail damage, powerful STAB Weather Balls, and slowly healing the damage it takes with its Ice Body Ability and the held item Leftovers.
When faced with its worst enemy, Fire-types, it can use Rain Dance to halve their Super-Effective STAB Fire-type moves, then use Weather Ball (Water) to deal Super-Effective damage, nearly TWICE as much as Water Pulse-actually MORE than twice, because Rain also doubles the power of Water-type moves, including Weather Ball (Water)!
If you already have Autotomize and Rain Dance set up, you can even afford to sneak in Weather Ball (Water) against Rock, Fighting, and Steel-types, for Super-Effective hits on Rock-types and neutral hits on Fighting and Steel-types!
In fact, some Rock-types are slow enough for an unboosted Vanilluxe to outspeed them and hit them with Weather Ball (Water) for huge Super-Effective damage!
This is the first and main reason why I go with Hail/Weather Ball/Rain Dance over the typical moveset of Hail/Blizzard/Water Pulse; slightly less damage from Weather Ball (Ice) than Blizzard in Hail in exchange for nearly double damage from Weather Ball (Water) than from Water Pulse when faced with Vanilluxe's worst enemies is a great trade-off in my opinion, great enough to make Vanilluxe way more versatile in battle!
Its major weakness is its lack of coverage; its only attacking move is Weather Ball, and by itself, it has access to three different versions: the Normal-type version, which activates in the absence of strong Weather and only has 50 Base Power; the Ice-type version, its main STAB move; and the Water-type version, its answer to its biggest threats.
Against the Water-type, which resists both Water and Ice, Weather Ball is effectively a 50 Base Power attack no matter what, but if Vanilluxe has Hail going, it at least gets STAB on the Ice-type form of Weather Ball, and some healing from Ice Body to mitigate the damage it takes from the painful matchup.
However, against some Dual-type Water-types, their secondary type will be weak to Ice (such as Water/Flying-types like Gyarados), Water (such as Water/Rock-types like Omastar), or both (such as Water/Ground-types like Swampert), so keep an eye out for opportunities to deal maximum damage.
What helps mitigate this weakness, and is the other reason why I prefer this Weather Ball build, is because while this wouldn't be the most powerful or self-sufficient sweeper, is because the Ultimate Vanilluxe is a great support Pokémon, especially for Weather-based teams, but also for others, such as protecting Grass-types with Rain Dance + Weather Ball, and getting their help against Water-types in return.
While I think it has good sweep potential in Singles due to Autotomize and being able to adapt to different situations, I believe it shines best in Battles with more than one Pokémon on each side, such as Doubles and Triples, as a Weather supporter that gains power from Weather-bringers like Politoed, Ninetales, Hippowdon, and Mega Charizard Y, and gives power to Weather-boosted Pokémon like Blizzard-users, Thunder and Hurricane-users, and other Weather Ball-users such as Castform.
Sunny Day/Drought makes Castform vulnerable to being OHKOed by Fire-type moves, even from non-Fire-types that don't get STAB, but also gives it access to Weather Ball (Fire), its Super-Effective answer to Steel-type opponents.
It can naturally outrun some Steel-types, and with one Autotomize boost, it can outrun many of them.
Sandstorm damages Vanilluxe, but gives it access to Weather Ball (Rock), its neutral-damage attack against the Water type, and shuts off Rain Dance, and with it, the Rain Dance + Swift Swim + powerful Water-type move + STAB + Rain boost OHKO sweep combo that many Water-types have access to.
In conclusion, this is my theory on the ultimate build for the Ultimate Vanilluxe:
But hey, that's just a theory...a Battle Theory!
Thanks for reading!