Hi Chris,
What you obtained is a selection of the pixels with grass. You are correct. This could be useful if you need to apply some treatment only there, for example.
As you mentioned, perhaps masking is what you need, although it can be difficult to differentiate between grass and vine rows. Soil and plant masking w/ Pix4Dfields
In our next release happening in June, you will be able to export a grid prescription taking Magic tool outputs too.
For now, you will need to draw each vine row as a boundary and then trim the layer.
I hope this helps.
José