I have a similar problem over in the Alaska maps. I have all the upgrades and towers.
ANK MK38 - Got
Caterpillar 745C - Got
Derry Longhorn 3194 - Still working on this one.
Hummer H2 - Got
Royal BM17 - Got
Yet I'm at 2/4 for Vehicles unlocked. I am confused. Shouldn't that be 4/5?
EDIT: I've now got the Derry Longhorn and am still at 2/4.