The first is the best choice because insert can figure out how many elements it's adding and resize the vector to fit before it starts copying. The others don't have that information, so could end up resizing after some copying, which would be slower than resizing at the start, or resizing more than once.
However, as @michaelgoldshteyn hints, since you're going to do two insertions, you can also resize the array yourself with the final size, potentially saving one resize.
