Explanation:

- Option 1: "Vini bought"

- This part is grammatically correct. It indicates that Vini purchased something.

- Option 2: "suitable for the festival"

- This segment correctly describes whether the dresses are appropriate for the festival.

- Option 3: "two new dresses"

- This phrase indicates the number and state of the dresses, and it's grammatically correct.

- Option 4: "but either is"

- This is where the error lies. The use of "either" is incorrect because "either" should be used only when referring to two distinct choices or alternatives. The sentence should use "neither" to convey that both dresses are not suitable.