At this stage, PyVibPTn is a thin wrapper on the VPT2 package inside the Psience library for vibrational spectroscopy.
PyVibPTn can be installed by downloading the source, however it will likely be easier (and more up to date) to use Psience, which is on PyPI and can be pip installed.
Please see the documentation for Psience and in particular the documentation for the VPT2 subpackage Psience for more information.