There are several places to find helpful Information regarding FD.io VPP.
VPP is part of the FD.io Linux Foundation project. The link describing the FD.io Linux
foundation project is here:
Much of the VPP developer documentation can be found on the VPP wiki.
Some of VPP is documented in the sources with Doxygen. See
https://docs.fd.io/vpp for links to Doxygen documentation by release.