My opinion on the whole Hope and Vanille thing was that they always had a close connection in the start, as in Hope clung on to Vanille like a scared boy and sought her for comfort (like when his mum fell to her death). During the vile peaks Hope began to draw away from Vanille in favour of Lightning who he saw as more strong and better to follow. When they eventually reunited, they still maintained a friendly companionship but it never got back to that initial dependency.

Will Hope and Vanille get together in XIII-3? Stranger things have happened, I wouldn't have called Lulu and Wakka having a baby together, so I suppose it's possible.