private static void writeVertex(final String vertexDirection, final Vertex v, final JsonGenerator jsonGenerator)
                throws IOException {
            jsonGenerator.writeFieldName(vertexDirection);
            jsonGenerator.writeStartObject();
            jsonGenerator.writeObjectField(GraphSONTokens.ID, v.id());
            writeLabel(jsonGenerator, GraphSONTokens.LABEL, v.label());
            jsonGenerator.writeEndObject();
        }
GraphSON serializers for graph-based objects such as vertices, edges, properties, and paths. These serializers
present a generalized way to serialize the implementations of core interfaces.

@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/graphson/GraphSONSerializersV4.java