Yes it means the gradient of f evaluated at the terminal point of the vector.
My equation for differentiability taken from the book is:
\lim_{(\Delta x, \Delta y)\rightarrow\ (0,0)}\frac {\Delta f - f_x (x_0, y_0)\Delta x - f_y(x_0,y_0) \Delta y}{\sqrt{(\Delta x)^2 + (\Delta y)^2}} = 0
If...