In this paper we study the existence of homomorphisms using semidefinite programming. Specifically, we use the vector chromatic number of a graph, defined as the smallest real number for which there exists an assignment of unit vectors to its vertices such that when . Our approach allows to reprove, without using the Erdős–Ko–Rado Theorem, that for the Kneser graph and the -Kneser graph are cores, and furthermore, that for there exists a homomorphism if and only if divides . In terms of new applications, we show that the even-weight component of the distance -graph of the -cube is a core and also, that non-bipartite Taylor graphs are cores. Additionally, we give a necessary and sufficient condition for the existence of homomorphisms when . Lastly, we show that if a 2-walk-regular graph (which is non-bipartite and not complete multipartite) has a unique optimal vector coloring, it is a core. Based on this sufficient condition we conducted a computational study on Ted Spence’s list of strongly regular graphs (http://www.maths.gla.ac.uk/ es/srgraphs.php) and found that at least 84% are cores.