How come Maladict gets to march during daytime in Monstrous Regiment without turning into ash?
Otto Chriek is also seen outside in the daytime (in the same book, among others) only turning to ash when exposed to the intense light from the salamanders. We know from Carpe Jugulum that vampires can train themselves to resist the usual ways of killing them, though the technique isn't perfect. Since both Otto and Maladict are Black Ribboners, this caused me to speculate that the vampire temperance league teaches the techhique to its members, perhaps as a sort of carrot: "give up the b-word and we'll teach you how not to burn in sunlight!"
Apparently it just being cloudy is enough to shield the average vampire, as this is part of the reason stated for Otto's survival walking around Ankh-Morpork, that it is, like England, typically overcast. Could be a similar situation in Borogravia.
Or perhaps they borrow sunscreen from the city's trolls, who originally couldn't walk around in the sunlight either.
When Vimes asks Sally a similar question in "Thud", she states that she's fine with long sleeves and a brimmed hat. Maybe it's direct exposure of sun to skin?
Well, it has been established that the Disc has as many kinds of vampire as tropical diseasesnote "presumably this means that some vampires are virulent and deadly, while others just make you walk funny and avoid fruit". It's conceivable that even without the training posited above, some would not have the vulnerability to sunlight.
While the chronic failure of the Borogravian military to catch on to its inflitration by females is justified by Rule of Funnyand the fact that so many of the brass are part of the Sweet Polly Oliver brigade, it's never said why its army's burial details would fail to notice that the fallen aren't always the right shape. They do strip their own dead for uniforms and equipment, after all, and Rule of Funny tends to lose its persuasiveness when senseless battlefield carnage is involved.
Presumably the burial details are also women infiltrators, or men who don't want to question a constant supply of replacement socks.
Fridge Brilliance: No wonder so many of them have risen to become the top brass. In a universe as troperiffic as Discworld, how could it be any other way?
Once a Sweet Polly Oliver gets killed in battle, there's little call to report her gender to anyone, as there's no point in drumming her out of the army posthumously. There is ample reason to keep quiet about it, as a unit commander who admits he'd had a woman among his troops and not noticed is bound to be punished for such ignorance, or at least branded too stupid to ever promote.