The sf.Vector2 constructor takes x and y, the two being optional and defaulted to 0. So if you write sf.Vector2(a) with a being a sf.Vector2, you'll get a as x. What you need to do is, unpacking a.
x, y = a
newVector = sf.Vector2(x, y)
Or shorter:
newVector = sf.Vector2(*a)
Good luck!