In 14w21b, starting with the layout in the first screenshot (and /gamerule randomTickSpeed 1000) will quickly fill in the corner as shown in the second screenshot.

Loading this into 14w25b and later versions initially loads correctly, but the corner vines eventually (probably as they're ticked for growth) will lose one or the other side, as in the third screenshot. Starting with the layout in the first screenshot in these versions will give similar results, the corner blocks don't have vines on both sides.