Loadiine loads assets from the SD card (it rewrite the console's firmware in memory in realtime to redirect WiiU NAND path to SD card path).
WiiVC is rebooting the console into vWii mode, cleaning from memory and exiting any running program in the wiiU mode (loadiine patches and wiiU firmware including), then the console emulate the Disc drive with the ISO included into the WiiVC.
The WiiVC could be located in WiiU NAND or WiiU HDD, so the WiiVC has the needed code to mount a encrypted NAND or wiiU HDD. it can't mount FAT32 SD or USB.
The vWii can't emulate the disc drive by loading the ISO from SD card. the ISO would not fit (4.7GB on fat32 is not possible)
the vWii launched in WiiVC mode is using a special coded wii IOS file, with its own drivers. it can't access USB at all.
Loading a WiiVC from loadiine is therefore not possible for multiple reasons :
1. loadiine is unloaded from memory
2. wiiu mode is exited
3. WiiVC code has only access to WiiU NAND or WiiU HDD.
4. WiiVC could be patched to access FAT32 devices, but it's hard to develop and not possible right now, and the ISO wouldn't fit on FAT32 anyway!
The best solution is to put the ISO on the console memory or on the console's formatted WiiU HDD. these are the only two locations available in WiiVC mode.
and, it's already possible to do it by injecting the any Wii ISO into a "host WiiVC", and installing it where you want (internal or hdd) then, loadiine is not needed at all to launch that game.
edit:
Oh, and another information, to explain why you can't do what you did before :
the Wii ISO (used by USBLoaders), and the Wii ISO (used into a wiiVC) are different.
the WiiVC is using a special format (new encryption), you can't extract these ISO and put them on your USB to load with USBLoaders.
you would need to use NFS2ISO2NFS to convert the ISO properly. but even with that, I'm not sure it would work.