Would something like this work?

Idea #1

public class VertexProperty extends Element {
    
    // Identifies the vertex that this Element's properties will be upserted to.
    private Pair<String, Object> vertexIdentifier;
    
    // Creates a VertexProperty with the vertex key and value used to find the vertex
    public VertexProperty(final String key, final Object value) {
    	this.vertexIdentifier = new Pair<>(
    		Objects.requireNonNull(key),
    		Objects.requireNonNull(value)
    	);
    }
    
    Pair<String, Object> getVertexIdentifier() {
        return vertexIdentifier;
    }
}