Suffer-Not-Injustice Vimes' existence was apparently common knowledge in Ankh Morpork education, at least according to Dragon King of Arms.
“Indeed. Ah-ha. Suffer-Not-Injustice Vimes. Your ancestor. Old
Stoneface, indeed, as he was called. Commander of the City Watch in
1688. And a regicide. He murdered the last king of Ankh-Morpork, as every schoolboy knows.”
Terry Pratchett: Feet of Clay
This would strongly suggest that Samuel Vimes' ignorance (in 'Guards, Guards') of both the existence of a Kingdom of Ankh Morpork and his own ancestor's part in King Lorenzo's downfall is probably best attributed to his own poor schooling, or possibly the fact that Dragon's knowledge of contemporary education is woefully out of date.
We can safely presume that the events of Guards, Guards and the persistent rumours of an extant King were sufficient to get adult Sam Vimes to show a greater interest in Ankh Morpork's history whereupon he discovered that his distant relative was 'Old Stoneface'.
We do know that at some point, Sam Vimes read about him in various history books.
“He, er, doesn’t appear much in the history books,” said Vimes.
“Sometimes there has to be a civil war, and sometimes, afterwards,
it’s best to pretend something didn’t happen. Sometimes people have to
do a job, and then they have to be forgotten. He wielded the axe, you
know. No one else’d do it. It was a king’s neck, after all. Kings
are,” he spat the word, “special. Even after they’d seen the…private
rooms, and cleaned up the…bits. Even then. No one’d clean up the
world. But he took the axe and cursed them all and did it.”
“I didn’t know this,” he said. “I thought there was just some wicked rebellion or something.”
Vimes shrugged. “It’s in the history books, if you know where to
Terry Pratchett: Men At Arms
We know that as an adult he went to the University Library and read his ancestor's journal.
Suffer-Not-Injustice Vimes wasn’t a pillar of the community. He killed
a king with his own hands. It needed doing but the community, whatever
that was, didn’t always like the people who did what needed to be done
or said what had to be said. He put some other people to death as
well, that was true, but the city had been lousy, there’d been a lot
of stupid wars. We were practically part of the Klatchian empire.
Sometimes you needed a bastard. History had wanted surgery. Sometimes
Dr. Chopper is the only surgeon to hand. There’s something final about
an axe. But kill one wretched king and everyone calls you a regicide.
It wasn’t as if it was a habit or anything…
Vimes had found Old Stoneface’s journal in the Unseen University
library. The man had been hard no doubt about that. But they were hard
times. He’d written: “In the Fyres of Struggle let us bake New Men,
who Will Notte heed the Old Lies.” But the old lies had won in the
Terry Pratchett: Feet of Clay
His trial was apparently quite famous, as were his last words.
Mrs Proust hesitated for a moment, and then said, ‘Well, if it’s true
about what they found in his private dungeon, then the answer is “yes”
in great big letters. They put the commander’s ancestor on trial
anyway, because chopping heads off kings always causes a certain
amount of comment, apparently. When the man stood in the dock, all he
said was, “Had the beast a hundred heads I would not have rested until
I had slain every last one.” Which was taken as a guilty plea. He was
hanged, and then much later they put up a statue to him, which tells
you more about people than you might wish to know. His nickname was
Old Stoneface, and as you can see, it runs in the family.’
Terry Pratchett: I Shall Wear Midnight