Not sure if about:blank works as a home page or not. But for sure vivaldi://startpage does not. It saves OK in the dialog, but gives an error whenever you push home page button. But then, if you press "enter" in the address bar while that error page is present, the speed dial appears. Weird.
Well, now that I try them both again in v1.0.156.2, I'll have to revise what I said above. I currently cannot get either about:blank or http://about:blank to work, even though http://about:blank worked in a previous version. (IIRC, it worked for me before without a Vivaldi restart, but An_dz previously reported it worked after a restart. In any case, I can't get it to work now even with a restart.)
As for vivaldi://startpage, that still "works" for me, but I have been testing it differently by hitting Ctrl+N to open a new window, rather than clicking the Home button. Yes, it should work with both, but now I'm getting the same results Ayespy described with the Home button. (I thought I recalled it was working before with the Home button in some previous version, but a quick check just now with 64-bit v1.0.142.32 yielded the same results as v1.0.156.2, so I'm probably mis-remembering.)