We are using two vMix instances ourself for production (one for the actual scene mixing producing the output with a german audio stream, and a second one for translater voice-over to produce an english-speaking version of the stream) and use NDI between those two vMix instances, too. It works in a very smooth way without problems, as NDI has very low latency and good quality. I guess your problem is your network bandwidth: NDI needs a stable and high-bandwidth connection. We use 1 Gbit/s connections and a Netgear AV-line switch. With this NDI works really in a smooth way without problems. Alternatively, if ultra-low latency is not really required, you can also try to send the stream with SRT from one vMix instance to the other.