why do you need to output them from vMix? they are already on the network as independent sources...

well...if you were to use the SDI/HDMI outputs of your LiveU and capture that as an input, vMix would then generate an NDI output - BUT - the NDI feed is a direct copy of the raw capture, and will not contain any color correction/audio processing that you may apply inside of vMix.

you can use the 4 output busses on the higher tiers of vMix (4k/pro/max) to process any input, incl NDI. Simply route a single input to each of the 4 output busses, and use the NDI feeds of the busses as sources for downstream gear like your switcher/replay. Only this workflow will give you the color correction/etc you apply on the vMix inputs. Audio can work this way too, by assigning each camera audio source to a different audio buss, and then associate the busses with their respective outputs to embed them into the processed camera feed. changing gain, dynamics and eq will all follow this way.

you'd need 2 vMix instances to process your 8 NDI feeds, but would def work. I still call this cost effective, you're basically getting 4 NDI processors (4k capable!) for $700+computer. each one of my standalone 1080 frame syncs/procs are worth far more.