[Micah]

No, you won't lose all your vamps to a mastery casting, but you're gonna lose about 20% of your horde per casting to someone with decent penetration. These traitor vamps will last for all of 1 spellcasting phase before being slaughtered by their former buddies since they're completely surrounded, so there's not going to be much of a tug of war even if you're firing off your own UM casts, just pure losses.

This isn't to say vamps are worthless, they're actually quite good at specific roles, but this is a gaping hole in the strat that needs to be plugged by AoL or antimagic in any serious engagement, and I think that leaving it out of the guide was a significant oversight that I wanted to address.

On the offense/defense debate it really depends on your position. In most cases when you're in a good position in the game you're able to dictate where major battles are fought (your victim's homeland), and thus have little need for defense, regardless of the value you can get for it. When you're overwhelming an opponent (and remember, fair wars are for suckers) the most you'll have to worry about is a few raiders which are generally better countered by spells than troops. On the other side of the coin, once you're being beat on by a larger opponent you're generally hard pressed to keep your blood flowing, and it's a bit late to invest in the vamp factory. So outside a scenario when you can dom-push well they're best in situations when you'll be fighting an even battle, or when you're expecting to get beat up by a larger opponent but have time to prepare. Neither of these comes up all that often, but when they do vamps are one of the perfect units to do some damage with.