Well, my mortar has been a killing machine lately, but after it got vet1 it doesn't get any exp from other battles, although it has killed 36 infantry in last 2 games.
I haven't tested it out, if it's the only unit that doesn't get any exp after vet 1.
Units dont get to vet 2 unless you purchase the vet 2 (and then later vet 3) upgrade.
Its currently not enabled, so, its stuck at vet 1 - and wont gain a point more once it reaches the threshold.
Unfortunately, Im dead on my feet after so much time working on this - I wont do it for another...12 hours or so.