sf::Vector2f and short are two conceptually different things. One is a 2D vector, the other a scalar integer.
So it highly depends what your member variables represent. From what you're writing, it seems like sf::Vector2f fits perfectly and short int would be a workaround. You won't gain any performance by doing so -- so make your program simple and optimize when you get stuck. Micro-optimization at this scale are a bad idea, unless you know exactly what you're doing and why one choice would be faster than another. Also, speaking about speed in terms of types doesn't make sense -- it's operations that take time, not types.