I assume you're talking about a vector field on $\mathbb{R}^n$ -- indeed, you likely mean a vector field on $\mathbb{R}^2$.
Assuming that's the case, the answer is that if the vector field $\vec{V}(x,y) = v_1(x,y)\hat{i} + v_2(x,y)\hat{j}$ obeys "The Canonical Flip on the Double Tangent Bundle" -- that is to say, if $v_1(x,y) = y$ -- then $\ddot{x} = v_2(x, \dot{x})$ is the second-order ordinary differential equation corresponding to $\vec{V}$.
(For an explanation of the correspondence between ODEs and vector fields, try Ordinary Differential Equations by Arnold. Also, many introductory texts on differential manifolds discuss how tangent vector field are [first-order] ODEs on manifolds; I prefer Differential Manifolds by Kosinski.
To understand The Canonical Flip on the Double Tangent Bundle, I recommend Transversal Mappings and Flows by Abraham and Robbin and The Geometry of Jet Bundles by Saunders.)