Using KJVS I click on Analyze for Gen 3:19. Go to "Define" and click on "bread" - I get the definition of the Greek (ie NT) words, not OT. Since I have a copy of Vine's on my MacBook, I know there is a Vine's definition for the Hebrew "lechem" (H3899), so why don't I get this?

That looks like an anomaly in the Vine's implementation. If you open up the VINE volume and look up "bread" you'll see two entries, one is "Bread" and one is "Bread (OT)" which is the O.T. definition. So our search on "bread" picks up "Bread" but not "Bread (OT)".