I was able to customize an ESRI light grey vector basemap in our Portal with help from this story map: https://www.arcgis.com/apps/Cascade/index.html?appid=a74a8b251f6141fa9561041d36ea56e3
I created a quick style with the Vector Style Editor and downloaded it as a json. In Portal, I added an ESRI vector basemap to a webmap (NOT as a basemap, just as a normal layer; Living Atlas content needs to be configured in Portal), copied it as a layer in the TOC, then on that new copy, clicked the 3 dot menu, clicked on Save Layer. On that new saved item in Portal, on its item details page, you can Update style with the json you exported from the Style Editor.