I see that Ubuntu Mate can manage with 512Mb of memory. The Vega has that and a Tegra 2 processor, but uses a touch screen. Is there any chance that it could handle Mate?
First of all, 512mb is the Minimum requirement, it doesn’t mean it will be an optimized experience.
Tablets are vastly different things than running distros on desktops or laptops.
Tegra 2 is an ARM cpu (ARM Cortex-A9) so it requires custom builds made for it, which at the moment do not exist. At the moment, Ubuntu MATE supports the Raspberry Pi 2 (ARM Cortex-A7) and is offering generic ARM builds for other developers to port it to other ARM chips.
Tablets also require ROMs because they use a different hardware system entirely so it is not as simple as booting an ISOs like it is for Desktops and Laptops.
Here is a website that might be possible to find custom ROMs for your tablet but I do not have one of those tablets so I can not test any of the content on that website for that specific tablet.