It's been a while since I programmed in C ( I now program in C++11 ).
If sfVector2f is a struct (which is probably the case), you can use an initializer list
sfVector2f Vector = { floatX, floatY };
for the other part:
sfCircleShape_setPosition(shape,(sfVector2f){wmode.width/2.0f - radius, wmode.height/2.0f - radius})
I can not tell. Sorry.
No, you can't instanciate C structures in the middle of an expression. Unless you create a function that does it.
sfVector2f vector(float x, float y)
{
sfVector2f v = {x, y};
return v;
}
func(..., vector(1, 2), ...);