I wouldn't call myself an expert on vertex buffers, but IIRC the OpenGL API to load data into a buffer just takes a pointer to the data and a size, so requires that your data is contiguous in memory beforehand - so deque may not have been an option.
Agreed. But I can't think of any data structure that would offer you contiguous, order-preserving storage and O(1) insertions/deletions of arbitrary elements.
There are actually implementations of deque which can store elements contiguously: imagine a vector where the "first element" is placed in the middle of the vector, and push_{front,back} reallocate whenever either end is reached. It wastes more memory than the typical deque implementation, but the elements are contiguous.
