Just for you to know, I sent a bricked-vWii Wii U last December for repairs with only like 5 days or so left before the warranty would expire. Here's my story.
The cause: I stupidly, ermm accidentally installed a Wii-only IOS80 version to my vWii. I've already had issues updating the Wii U's system. I was suspicious it was related to a patched IOS inside the vWii, since from what I read, this new update somehow was trying to either install, hash check and/or update some IOS(es) inside the vWii, since once updated, it added a couple of new enhancements to the vWii. Also, without this update you were completely tied hands and feet down by not being able to connect to Nintendo's servers on the Wii U side. This meant NO online activity whatsoever; eShop, Netflix, playing online, etc., etc. This update was mandatory! If you wanted to do something as basic as to be able to connect online, you needed to update, period! Back then, this was something totally unheard of, never before had an update affected the vWii side. And so there was very little, almost non-existent info on the matter. Nonetheless, I overread this one thread that, had I read it thoroughly, it would have given me the answer to my prayers.
So the first thing I thought was that perhaps my vWii's menu IOS80 was the culprit since I remembered patching it in order to be able to install and run all the Wiiware and Virtual Console goodies I had into the SD channels section within the vWii's Wii menu (NAND). So it became clear to me that what I needed was to reinstall a clean IOS80. Spontaneously, almost like magic, the NUS downloader app popped into my head, it had saved me before when I used to own a Wii. You see, NUS and I, we both share a story of our own, having fought together in countless great epic battles and tasted the flavor of victory with triumph after triumph back in the good all days, hence somehow, something inside me really wanted NUS back. It was telling me "go NUS" but actually little did I know I was on the path to going "nuts". Then I had this deja vu momentum where I was convinced that I had seen a vWii section within the NUS downloader app sometime, somewhere before; thus I was certain NUS was also vWii compatible; it isn't! it wasn't! it has never been nor will it be!
To add to the confusion, before installing this new clean IOS80 provided by NUS, and as a precautionary measure, I decided to go on and compare my patched vWii IOS80 version with NUS' latest IOS80 version using Wii Mod's IOSs managing feature. I selected my installed-and-patched vWii IOS80 and focused on the line that said "Latest: v6944", it was the exact same version as the NUS IOS80's. Even though there was this other line that said: "Version installed: v7200" I rested on the fact that v6944 was the latest and since it was mentioned here inside the properties of my vWii patched IOS80, it had to be vWii compatible... boy was I wrong. But really, the bottom line is: I don't know why on earth I thought IOS80 was a vWii exclusive IOS. Bad assumption! It just so happens that there is an IOS80 for the Wii 4.3's system menu as well! but with a different, lethal, non-compatible version that will undoubtedly brick and bring to its knees even the strongest of vWiis out there.
The outcome: Long story short, either Nintendo couldn't access the vWii mode, didn't want to, or they were just in a hurry to dispatch all queued repair orders... either way, a few days later, I was shipped a brand new-refurbished unit. Call it a stroke of luck but I guess I'll never know if in the end I was just a very lucky chap or if there was really no way for them to find out, who knows?!
My two cents