Just the idea of this lawyer having a trained monkey who is really the devil is kind of comical. Imagine the devil masquerading as a monkey in a valet's uniform, it's kind of loony. It's a lot like the absurdist humor of today, like those Burger King commercials with "the King" appearing in someone's bed to present him with a bacon and egg sandwich. So if Vladimir has that modern sense of humor, absurdist, I can see why he thinks it's funny. I have a bit of that myself, having watched so many dumb movies.
The priest's cry of "Infernal beast!" is also funny from a certain mindset, this priest screaming at a seemingly harmless little monkey. If you put that in a movie it would bring down the house.
I read the same story -- yes, it is in modern editions -- and I found it more horrifying, it reminded me of Edgar Allen Poe stories like Hop-Frog or Rue Morgue ( which possibly drew on this very tale from St. Alphonsus, since Poe drew on lots of Catholic material ). But it depends on your mindset when you're reading it, I can see how it would strike someone as funny.