I do understand where you are coming from Rob, since I found myself confounded with a similar problem, in both AgeOD games and in other grand strategy series. Sometimes, it just becomes too overwhelming, from the amount of units (which only become more difficult the more you advance in the timelime), from the size of the territory. Since Vainglory is being set on the largest level ever seen in an AgeOD game, with complexity to rival any other Paradox game (yes, I went there), I can see the doubting prospect you have in front of yourself.
If you would allow me to offer one small piece of advice, it would go like this. Take your time. AgeOD games aren't meant to be played quickly, or without, well, strategy. I find it easier to spend 5 or 10 minutes just running across the screen, finding all the units, understand what is being offered to me by a particular scenario and then planning my turns. Even restarting the same scenario, one or two times, is a very acceptable tactics. No need to fear losing the war, you can always go back.
So, next time you have difficulty, just remember. Take your take, take a good look and most importantly, contrary to some other series, you do not need to micromanage your entire territory, merely the front lines (in most cases). In the end, with VoN and other similar games, you must control a large empire, which takes a lot of time. So better to invest it in the very beginning, than to lose yourself at the end. You'll find the experience very, very rewarding and is one of the reasons why I enjoy coming back to all the different titles this magnificent game company has to offer.