Publishing a vector tile package to Portal for ArcGIS does not consume credits in any way. In that setup, the vector tile package becomes a vector tile layer served out entirely from your own infrastructure using the full ArcGIS for Server Web GIS stack: Portal for ArcGIS, ArcGIS Server (in the role of a hosting server), and ArcGIS Data Store.
If you publish a vector tile package to ArcGIS Online, it is served out from there. A small amount of storage credits are charged, but no consumption credits apply.