I guess Martin feels audio renders so quickly that to edit the clip offline in Audacity or whatever is almost as quick as setting in and out points.

Or if you don't want to do that, you could switch off "automatically restart with transition" and queue up the audio clip in advance and listen for your end point.

There is a feature request for this somewhere which I also can't find at the moment because I can't search for my own posts (my username is only three characters which means I can't search for my own posts).

Sadly not: "Search texts must be at least 4 character long and compliant with forum rules for search (see forum FAQ for more info)."

Why don't you play this audio file over the network then ? You could use VLC as a player (it is easier to find mark in) and use NDI to feed vMix. Then you just have to switch off this input for mark out...
I'm sure there are better options like using Dante or else but this should work.
Remember vMix is a LIVE switcher. Operating vMix underlies being able to operate it live for any type of source. There is not much difference into airing an excerpt, a portion of a sound file or switching the right angle from a multicam setup.