There's no such thing as 'O(n/2)', big-O notation is about worst-case complexity. O(n) means 'linear time in the number of elements', not 'n operations' or something.
But in practice, the amortized time complexity will indeed usually be half of the worst-case scenario when removing elements from a vector.
But in practice, the amortized time complexity will indeed usually be half of the worst-case scenario when removing elements from a vector.