(Ill-recalled story is that in the 18th century it was customary to insert an improvised violin solo in the middle of chamber pieces-- or was it symphonies? Anyway, one evening a Mr. Farrell was the violinist, playing with Haydn, and having modulated (if that's the term) out of the key of the work during his improvisation, couldn't get back in. And played and played and modulated and modulated and *finally* got back to the right key-- at which Haydn stopped conducting, came over, shook his hand and said 'Velcome home, Mr Farrell, velcome home!')
Vetinari was never sure. Of course, the physical resemblance between John Keel and Sam Vimes was unmissable, but that was no cause for surprise. Vimes was born and bred in the Shades, where family lineage tends to be less a matter of 'four quarterings or eight on the escutcheon?' than eeny-meeny-miney-mo among four possible candidates for one's father. Or in some cases, eight. People travel between Pseudopolis and Ankh-Morpork all the time: it's hard to find a good seamstress in Pseudopolis. Add to that the, well, frankly paternal attitude Keel was said to show young Vimes, and the conclusion was obvious.
Vetinari stashed the little semi-fact away in the thick file at the back of his brain labelled 'possibly useful?'
Then there was John Keel's sad and puzzling death on the glorious 25th: the death Vetinari was supposed to have prevented and failed to. (His aunt had been extremely displeased.) Vetinari was even less sure about that one. Death does indeed change people. A body in its coffin, replete with all the devices an undertaker can think up, looks so little like the man when he was alive that it's a wonder the family doesn't revolt-- 'That's never our Da!'-- and refuse to pay his fees. So if John Keel dead (pale, grey, covered in blood and what Vetinari's anatomical training insisted were days' old wounds) looked nothing like John Keel alive (dynamic, energetic, one mean bugger), was it to wonder at?
Vetinari stashed the little niggle away in the thick file at the back of his brain labelled 'anomalies.'
And there it sat, while Vetinari worked to bring order out of the city's chaos, or at least to make the chaos more efficient, and observed Samuel Vimes' slow rise to captain of the Watch and concomitant slow descent into alcoholic befuddlement. A pity he didn't have his father's energy and drive, Vetinari shrugged to himself. Unless Keel's energy and drive itself had been a product of circumstance...? He'd been a good determined Watchman while in Pseudopolis (Vetinari sent someone to look up the files there) but nothing suggested any genius for organization or personal charisma. That had clearly been called forth by the events of the May Revolution. A similar upheaval might have the same effect on Vimes.
Several times over.
Chip off the old block, Vetinari thought. Blood will tell. Circumstances make the man, as they say.
Much too neat. I don't believe it for a minute.
And a little niggle in the Anomalies file coughed and said "Erm...?"
Vetinari looked at it and said, politely, 'I'm not sure I can believe that either.'
So it was extremely satisfying, that May evening, when his solitary reminiscencing in Small Gods was interrupted by the sight of Sir Samuel Vimes grappling with a known murderer and subduing him with his bare hands. (Vetinari had a thin knife ready in case the situation showed any signs of turning into 'failing to subdue.') As Vimes stood up afterwards, all bloody from the struggle, Vetinari found himself looking straight at the face of John Keel. Delicious, almost orgasmic, the scratching of an age-old mental itch. So that's why, he thought contentedly, and deleted the little niggle from Anomalies and the little semi-fact from Possibly Useful.
And added a very large HOW? to the file labelled Think About This.