Yes, 1TB works on vWii (even bigger drives).
I don't know wiiflow and its usual issues so I can't help you with this error message.
USBLoaderGX doesn't run? what do you mean? nothing on screen at all? just black screen? it doesn't even launch?
Or it launches, displays text but freeze ? (known issue with some HDD)
Attention, DO NOT FOLLOW THIS ON vWII :
This is for Wii only.
On your wii you already have HBC installed or not?
if you have HBC installed, the steps for cIOS installation are the same, but with a different installer version (one
for Wii instead for vWii). And the NAND dumping is done from Bootmii interface instead of using a homebrew launched from HBC.
If you don't have HBC, you will have to follow the same step you did to install it on vWii.
If your HBC is not running on IOS58, You might want to update/install IOS58 before re-installing HBC if you didn't update your system menu for a long time. (it's not needed to update, you can keep it on low version but
install IOS58 and reinstall HBC)
just be sure not to mess the cIOS from wii and vWii. It's recommended to use a different SD card to be sure it's not using cached files from the other console.