On one side we have : take Vanheim with a W9F9 bless and produce Vans, disregard everything else, you'll expand lightning fast, then win 95% of fights vs any comparable-cost army, plus you can raid, hide, attack when and where you choose, play tricks to your leisure.
On the other we have : take another nation, consider carefully how to make an army that could be effective vs vans, how to script it, what spells to use, etc etc..
And maybe if Vanheim is cool he'll fight on your terms,and you'll win. Maybe
And that'll be "balanced" ? C'mon....