You are confusing two different issues: the Grass plugin (Joseph answer and the new GRASS Plugin ready) and Grass in the Processing Toolbox:
- with the GRASS plugin, you work in GRASS GIS and you must use v.in.ogr to import your shapefile in the original GRASS GISDBASE (locations, mapset) -> The Joseph answer
- a GRASS script in the processing toolbox creates first a temporary GRASS GISDBASE, import the layer with v.in.ogr, do the work and finally destroy this temporary folder (see How to modify existing grass gis tools and run them in QGIS?How to modify existing grass gis tools and run them in QGIS?)
In theory, These GISDBASEs have nothing to do with each other but if you know Python, you can modify the procedure.