Iβm starting to realize that I misread Feet of Clay when I first read it and just carried that misreading with me until now.
In the last bit of the book, Vetinari gives the poisoned candle to Drumknott, revealing that he had learned about the plot early and had been feigning his symptoms to keep Vimes prodding into the heralds and the guilds, while actually snipping off the candles to make them look like they had burned down to stubs.
I misread this that instead Vetinari had learned about the plot and was still snipping off the poisoned candles, but was still burning enough of them to poison him a little bit. Enough to keep up the symptoms, but never enough to actually kill him. As a treat, I guess. For fun.
And when I first read the book Vetinari read to me as such a little freak (affectionate) that I never thought to question this until now