I can't find the option to delete a Virtual field on a layer, not even in the Table Manager plugin.
Where is that option?
You can use Delete column tool available from attribute table. It works just fine with QGIS 2.8.2.
One tip for anyone struggling to permanently delete a virtual field:
I'd deleted my virtual fields numerous times using the Delete Column tool in the attribute table, as well as in Layer Properties-->Fields, and while my virtual fields went away immediately, they always reappeared for some reason, especially when saving new versions of my layer (the new layer was repeatedly saved with the virtual fields in it).
I was able to stop this by deleting the fields via the Refactor Fields tool, which creates a new layer without* them reappearing. Hope this helps somebody as bewildered as I was until about 5 minutes ago.