To install manually against a Slicer build:
SLICER_BUILD_DIR=/path/to/Slicer-SuperBuild
git clone git://github.com/vmtk/SlicerExtension-VMTK.git
mkdir SlicerExtension-VMTK-build/ && cd $_
EXTENSION_BUILD_DIR=`pwd`
cmake -DSlicer_DIR:PATH=$SLICER_BUILD_DIR/Slicer-build ../SlicerExtension-VMTK
make -j5
make package
$SLICER_BUILD_DIR/Slicer --additional-module-paths \
$EXTENSION_BUILD_DIR/inner-build/lib/Slicer-4.3/qt-loadable-modules \
$EXTENSION_BUILD_DIR/inner-build/lib/Slicer-4.3/qt-scripted-modules