Now Vials has two advantages:
1.stakable: It can conserve bag room
2.quick drinking: Faster than normal potion bottle,about 1 second to finish this action

But Potion Bag and Potion Rings can also provide these two advantages
Potion Bag: Can contain more potions,can right click to quick use potions in bag
Potion Rings: Controllable.Can be quick activated and deactivated,can conserve your material（gain xp is easy after I have a xp farm）

Also Vials has two disadvantages Bag and Rings dont have:
1.consume more material: It need more glass and powder to craft it
2.inconvenient crafting measure:the splash potions are unstakable,this makes crafting not so easy (bows also makes crafting dispensers troublesome)

The problem above makes vials can be easily replaced by other two and showing a little bit useless in game.
As for vials' item icon, I thought it can bottle the potion liquid separately,For example,I can choose to drink a normal bottle to gain 8 min buff, I also can separate them In two vials so I can use them two times to get shorter duration.but it doesn't, so I don't think vials are alternative enough in game.Vails can be better if it's low consumption in carfting

I agree.
I was actually debating if I wanted to remove vials altogether.
The behaviour you're describing was actually in the mod in the past, in the form of potion combining and splitting, and required no extra item, vanilla potions were enough.

I'll think of some other way to make them useful again, or remove them if no good option comes to mind

Yes,It is.
vials in reality also play a role of combining and splitting,this may limit better options.
Maybe you want to avoid repeating with other mod.but I think short time buff is still needed for short time battle,combining and splitting is also a great way to increase player's feeling in operating liquid.In your mod,breaking vials is really cool I think :)
BTW,milk of buckets occupy too many room,maybe fill in vials is a great idea.

I think they could be made useful as a defensive tool against other potions. So you could make it so it can pick up the splash potions of others, or take an effect off of yourself, to store for yourself.