Enlarge/Reduce does nothing to change the damage of Vial of Acid.
The target’s size doubles in all dimensions, and its weight is multiplied by eight. This growth increases its size by one category– from Medium to Large, for example. If there isn’t enough room for the target to double its size, the creature or object attains the maximum possible size in the space available. Until the spell ends, the target also has advantage on Strength checks and Strength saving throws. The target’s weapons also grow to match its new size. While these weapons are enlarged, the target’s attacks with them deal 1d4 extra damage.
If you use Enlarge to enlarge a vial of acid, all it does is make it bigger. That’s it. It doesn’t change the damage, because nothing in the spell says it changes the damage. See here for more details on “spells only do what they say they do”: What is the source of the “spells do only what they say they do” rules interpretation principle?
This seems pretty intuitive from a balance perspective. Vials of Acid are 25 gp, Enlarge/Reduce is a 2nd level spell, and catapult is a 1st level spell. Combining these very early game features shouldn’t produce an effect more powerful that many 6th and 7th level spells.
Using catapult can deal 4d8, but doesn’t activate the acid.
Catapult, which has a casting time of 1 Action, says:
If the object would strike a creature, that creature must make a Dexterity saving throw. On a failed save, the object strikes the target and stops moving. When the object strikes something, the object and what it strikes each take 3d8 bludgeoning damage. [Up casting to 2nd level adds 1d8 damage.]
However, Vial of Acid says:
As an action, you can splash the contents of this vial onto a creature within 5 feet of you or throw the vial up to 20 feet, shattering it on impact. In either case, make a ranged attack against a creature or object, treating the acid as an improvised weapon. On a hit, the target takes 2d6 acid damage.
These are two entirely different features that both require an action and cannot be used at the same time. You cannot both catapult and get the acid damage. You must choose one.