My apologies, I do not have experience with OwnCloud Server, so I can not offer any advice for that.
Dual-Boot firmware is available for the VIM2, see here for image. It is older, however, Khadas Team member hyphop has said a new dual-boot firmware will be coming for all the VIM model(1, 2, 3). You may want to wait and see. The current dual-boot firmware is for USB flashing methods and the flashing programs for Linux and Windows.
I mentioned Krescue before. Krescue can be used to download and install certain images. Benefit? Other than making the Krescue uSD card, no PC is needed. It is easy and self-contained.
The firmwares Krescue can install can be see here. Once hyphop releases the new dual-boot, it will be found there.
Otherwise, Ubuntu can be installed to the VIM2 or run from SD/USB, using the usual methods.