OK, so I decided to copy my post from here to a thread of its own in an attempt to highlight what is more or less safe to do on a Wii U's vWii WiiMode, and what is not safe to do, to try to raise awareness and educate users to try help prevent them from bricking their WiiMode. Keep in mind though, that this list is only really based on my experiences with vWii, and a few other experiences by other users too.

Maybe this could be useful enough to get stickied?
Wow, it did get stickied! Thanks!

So, here is my basic list thus far:

Safe (should be ok to run without worry):
- homebrew that does not install anything or require installation, or even better, ones that do not write to NAND at all
- installing The Homebrew Channel. While this does install to NAND, this is the most reputable of all Wii homebrew launchers, and the authors (Team Twiizers) make sure it is as safe as possible before they release it.

Potential risk of brick but still more or less doable (make sure you have a NAND backup, or at the very least a key dump from the modified xyzzy + hardware programmer):
- Installing channel wads (same reason as on regular Wii, mainly only if they have a bad banner or something, otherwise if they are confirmed ok, then they should be safe)
- Installing non-critical system parts, like game IOS, patched IOS, or cIOS into unused slots (such as d2x v10 vWii version), as long as one is installing vWii versions only on vWii, and regular Wii versions only on regular Wii (they are not used by System Menu and should be fine to remove and reinstall if you screw them up)
- Modifying 1-513 "BC-WFS". According to crediar, this is used by vWii to be able to load up and give compatibility to Dragon Quest X. Dragon Quest uses it's own "WFS" formatted USB drive, so that is probably why. BC-WFS is encrypted, so it cannot be successfully modified until the encryption is publicly cracked. But messing it up should not harm anything on vWii other than Dragon Quest X.

Will brick if you do it wrong, you can do these BUT only do if you know EXACTLY what you are doing: (a NAND dump, and at the very least a xyzzy key dump + hardware programmer will be the ONLY thing to save you if you screw up, due to no bootmii as boot2 on vWii)
- Modifying System Menu IOS in any way (is currently IOS80)
- Modifying 1-512 "BC-NAND". According to crediar, this title is needed by vWii to be able to load NAND titles, like channels. Messing this up would break channel loading, which is no good, but disc games should still work. Breaking this would be harder to fix due to the inability to load channels. It is also encrypted, so cannot be successfully modified until the encryption is publicly cracked.

DO NOT do, this stuff is just plain stupid and confirmed as not working and brick causing:
- Following a WII softmod guide. WII and vWII ARE DIFFERENT ! DO NOT USE MODMII ! FULL BRICK ASSURED
- Installing regular Wii IOS on vWii. Installing non-critical IOS will result in those IOS having very limited functionality and crashing very easily, installing old System Menu IOS (IOS 80 atm) will cause vWii to brick.
- Installing IOS downloaded from NUS with NUSD. NUSD will not sign/encrypt the files correctly.
- Installing IOS extracted from NAND dump. Repacked wads are faked signed.
- Installing vWii IOS on regular Wii. This actually seems worse than the inverse. While Wii IOS maybe actually load up on vWii but still crash when trying anything, vWii IOS on regular Wii will not work at all. Also, users have reported very strange Wii bricks (among them being myself and FIX94) from installing vWii IOS on them, even non-critical ones... I have no clue why they do this, but new IOS for new hardware, there has got to be a reason somewhere.
- Installing vWii IOS wad files on vWii packed from a vWii NAND dump using a program on your PC. The program used to create the Wad file is fake signing the ticket. While it install fine and seems to works fine on vWii, it bricks the Update feature of the WiiU Mode, and prevent reinstalling The Homebrew Channel on vWii!
- Installing IOS, or deleting IOS that you do not want anymore, or re-installing a WAD version of the IOS if you didn't use YetAnotherBlueDumpModto create it.
- Modifying vWii System Menu in any way, including priiloader or "themes". Do not modify the System Menu's .dol, it has extra encryption that has not been publicly cracked yet. Also cannot install priiloader for the same reason. Since vWii System Menu is not the same as regular Wii's System menu, regular Wii "themes" are not compatible, and will brick vWii. AFAIK there are no vWii themes released at this time, and I myself am unsure about making or installing my own myself, as I have not tried it yet.
- Deleting The Homebrew Channel if it doesn't work anymore. The homebrew channel is not working because you messed the IOSes. The Homebrew Channel is working fine, do not ever delete it! it's your most easiest way to restore and fix a bricked vWii. If you delete The Homebrew Channel, it will be harder to fix your console.
- Deleting Forwarder Channels if you bricked your vWii. Same as The Homebrew Channel, a forwarder can be found useful to unbrick your console. Don't delete ANYTHING if you think your console is not working. Ask to other users on the forum first. It will save your console.
- Trying to do or install anything at all if you do not know what you are doing, seriously, read up and learn about everything before attempting to mod anything, this should be a given!

What is recommended to do:
- Install The Homebrew Channel.
- Then run the modified xyzzy v1.2.1 unnofficial provided by DarkMatterCore on your vWii via HBC. It will dump your vWii's unique per-console encryption keys that are needed to decrypt and re-encrypt a dump of your vWii's NAND on PC, useful in case if you brick vWii, you can (or send it to someone who can) dump the vWii's NAND into PC using a hardware programmer, like an Infectus (no longer manufactured) or Progskeet, and then decrypt it to alter it and fix it, then re--encrypt and reflash the fixed NAND dump onto the vWii's NAND chip. Especially useful due to there not being any vWii version of BootMii.
- Dump your IOS using YetAnotherBlueDumpMod to wad files and answer NO when asked to fakesign the tmd or the ticket to keep a CLEAN version of each IOS you own. When vWii system is updated, remember to redump the modified IOS to keep latest version of clean IOSes. (you can check wiimpersonator to see modified titles). Clean IOS wad files can be used to fix semi-brick vWii as a full NAND restore is not possible ! Keep them safe. More info here.
- Educate yourself on whatever it is you want to do before doing it, do not just follow guides without learning about what each part does. Be lazy and you could pay the price for it someday!
- Again, only mess around with important stuff if you know EXACTLY what you are doing beforehand. It is your own responsibility if you end up bricking anything. As you can see above, there are several things, that if you break them, can only be fixed with hardware programmer + keys from xyzzy.

Now do keep in mind though, if you DO brick your WiiMode vWii, Wii U mode will still function fine, you will just no longer have any Wii Backwards Compatibility though. But still, hopefully this still does not happen to you.

Long story short, all that is needed is some education, plus awareness of what could happen, and some common sense.
Having those unique per-console keys helps too
Especially since: performing any sort of "unauthorized technical modifications", AKA any of this, will void your console's warranty! Remember that too.

Update (2016-12-17):
A While back, @Mr. Mysterio made an app that formats SD cards properly for vWii exploits. You can get it here:
http://gbatemp.net/threads/wii-u-homebrew-with-4gb-sdhc-card.386419/#post-5444111

Program Instructions (provided by Mr. Mysterio):
1: Open makefat16.zip and run makefat16.exe
2: Type the letter of the drive to format.
3: Check the properties of the drive in My Computer. It should show a 2GB FAT16 drive!