Let $G=(V,E)$ be a directed graph. Let $E$ be a binary relation such that $(x,y) \in E$ iff there is an edge from vertex $x$ to vertex $y$.
Let the world of first order interpretation be the set of vertices. A vertex cover $C \subseteq V $ of a graph $G=(V,E)$ is a set such that
$$(x,y) \in E \Rightarrow x \in C \lor y \in C.$$
How would I prove that the set of all first order interpretations that have a finite vertex cover is not definable.
Thanks in advance.