[Alneyan]

Usually you do not get one Vine Ogre per casting, but two: the Ivy Crown replica gives you one more Vine Ogre per summoning, and you should have one of those items per Vine Ogre summoner if you are serious about it. Vine Ogres seem cheap enough at 0.5 gem the unit, if you have enough mages to summon quite a few of them.

Vine Ogres are especially appealing because Nature does not really shine for the unique summonings, unlike Air for the Air Queens, so you are more likely to spend your Nature gems early on rather than those air gems (if you are trying to snatch an Air Queen). That, and Corpse Constructs (the closest equivalent to Vine Ogres in spirit) require harder to get magic, while not really being on par with the Vine Ogres, and that's only if you find enough Air-1/Death-1 cheap mages... or even *one* source of cheap Air 1/Death 1 mages, Return of the Raptors excluded.