This is not a native effect function in Vmix. Has anyone tried to do this through some sort of app? I have not found anything Are there any recommendations on an app, or editor that can do this for live video? Thanks.

If I've understood you correctly, this is actually quite easy to do in vMix.

1) Add a Mix input

2) Put whatever source you want to echo into that Mix

3) Set that Mix input as a spare Output (say Output 4) and turn on NDI

4) As a separate input, add the NDI source of Output 4

5) Delay this new input by, say, half a second

6) In a new blank input, add the original Mix input, and the NDI input, as layers 1 and 2, and adjust opacity of each to suit.

7) Put this new input into PGM