Please note that the wrapper concept from NASL side around such external tools like wapiti is basically deprecated and won’t receive any updates / fixes from Greenbone side. The mentioned wrapper VT is currently only kept in the Greenbone Community Feed for legacy reasons.
Calling such external tools should be done via an OSPd wrapper (https://github.com/greenbone/ospd) where you can find some basic documentation on how to implement one at https://github.com/greenbone/ospd#how-to-write-your-own-osp-scanner-wrapper
For further questions around OSP the existing category About the Open Scanner Protocol (OSP) category could be used.