Since Valyrian Steel weapons have some special enchanting rules and only count as +1 weapons for the purpose of determining the costs of adding Enhancement bonuses, we can put the Undead-, Fey-, and Winter-Bane effects on a VS weapon for 3000 IM in enchanting, making them the equivalent of +4 weapons.
Damage-wise, that translates to Weapon Damage (lets use Greatswords for this example; 2d6) + 2 (VS Enhancement) + 6d6+6 (Bane Effects) = 8d6+8
That 8d6+8 is base damage, before the wielder's Strength bonus, class features, feats, etc., are applied.
Assuming a 6th level Fighter with a Strength of 18, Weapon Specialization and Weapon Focus (Greatswords), that's another +8 damage.
And then, because I'm a completionist, let's replace the VS Otherbane Greatsword's base +2 Enhancement with a temporary (20+ hours) +4 Enhancement thanks to a Magic Army spell being cast by Viserys, Lya, etc.
Now our hypothetical 6th level Fighter is going to have an Attack Bonus vs Others of +21/+16 and each successful attack will inflict at least 8d6+18 damage.
Of course, that's too much of an investment to make in a measly 6th level mook during a big battle vs the Others, but it's still a pretty neat way to show how effective stacking the enchantments would be.