Has anybody done this just as an item?
I used the following workflow (assuming you have your own portal as well as AGOL org account):
1. save esri vector basemaps as webmap items to your own AGOL org account. Here is a list of all esri vector basemaps: http://www.arcgis.com/home/search.html?q=owner%3A%20esri_vector&t=content&focus=maps
2. use https://ago-assistant.esri.com/ to copy saved webmaps from your AGOL org account to your portal
3. share these webmaps as basemaps in your portal
Retrieving data ...