If the property 'vertexFormat' is an attribute, e.g.
<TriangleMesh vertexFormat="POINT_NORMAL_TEXCOORD">,
TriangleMeshBuilder works fine.

If it is an element a ClassCastException will be thrown
when the following test case is loaded into the 3DViewer
(com.javafx.experiments.jfx3dviewer.Jfx3dViewerApp).

VertexFormatTestCase.fxml

<?xml version="1.0" encoding="utf-8"?>

<?import javafx.scene.shape.MeshView?>
<?import javafx.scene.shape.TriangleMesh?>
<?import javafx.scene.shape.VertexFormat?>

<MeshView xmlns:fx="http://javafx.com/fxml">
  <mesh>
    <TriangleMesh>
      <vertexFormat><VertexFormat fx:constant="POINT_NORMAL_TEXCOORD"/></vertexFormat>
      <points>0.0,0.0,0.0, 2.0,0.0,0.0, 1.0,-2.0,0.0</points>
      <normals>0,0,-1</normals>
      <texCoords>0.0, 0.0</texCoords>
      <faces>0,0,0, 1,0,0, 2,0,0</faces>
    </TriangleMesh>
  </mesh>
</MeshView>

Caused by:
java.lang.ClassCastException: javafx.scene.shape.VertexFormat cannot be cast to java.lang.String
at com.sun.javafx.fxml.builder.TriangleMeshBuilder.put(TriangleMeshBuilder.java:106)
at com.sun.javafx.fxml.builder.TriangleMeshBuilder.put(TriangleMeshBuilder.java:37)
at javafx.fxml.FXMLLoader$PropertyElement.set(FXMLLoader.java:1413)
at javafx.fxml.FXMLLoader$ValueElement.processEndElement(FXMLLoader.java:790)
at javafx.fxml.FXMLLoader.processEndElement(FXMLLoader.java:2823)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2532)