thanks all for your aswers,
I would just pick one of the vector types that feels more comfortable to use or where you estimate less conversions would be needed if you need to call one of those librarys more often. Later if its really too slow you can always profile and see if you need to improve this or something else.
i think i will chose this way and try to separate between gameplay object class and graphics class, in my problem i will use b2vec2 more because my game using box2d to program its gameplay and create a converter function for it for sfml related use. well, hope it's the most efficient way
Both Box2D and SFML are open source. Recompile one of them to use the other's vector.
this a good idea but, it'll be a problem with me since sfml using template to define it's vector and i'm still don't know how to use it. moreover b2vec2 using struct to define it's vector and of course it'll make me confusing