Dio recruiting Jack the Ripper and vampirizing him was just a way to further amplify the magnitude of Dio's evil. Namely, he got one of the scariest people in real-life England to join his side. The battle between Jonathan and Jack ends with Jack being disintegrated by hamon force. This doesn't just explain why Jack the Ripper's killings ended without an arrest. It's also, in Jojoverse, why Jack was never identified with anyone's birth name. How do you identify someone who now only exists as a fine film of dust?
The name of Jonathan's sword, Luck & Pluck, reflects both his hard-working nature and the insane luck he required to succeed against superhuman adversity even before his own abilities became superhuman. Especially since his nemesis was Born Lucky.
To give the sword the name Luck & Pluck, Bruford added "P" to the word "Luck". Since that moment Joestars didn't have a shortage of pluck, but get a strange tendency to encounter (and sometimes fall victim to) dangerous maniacs with supernatural powers. Could it be that correcting "Luck" into "Pluck" actually took the luck away, while giving bravery?
The only way you can finish off a vampire with Hamon is to cause it to reach their head, and Dio normally drains people by jamming his fingers into their head. Now think back to that boxing match Jonathan and Dio had in their childhood; the rule was that the first hit in the head decided the match.
The opening sequence of "Sono Chi no Sadame" begins with manga screenshots of the first six JoJos, going from Jolyne to Jonathan. But why start with Jolyne when, at the time of the Phantom Blood anime's premiere, 2012, Part 8 was already in full swing, and thus we had eight JoJos? Well, given that the title of the song translates to "Destiny of that Blood", it's only appropriate to show only the JoJos directly descended from Jonathan, which chronologically ends with Jolyne. Being JoJos of an alternate universe, Johnny Joestar and its version of Josuke Higashikata have no lineage at all from Jonathan.
In a case of heredity, Jonathan's Sunlight Yellow Overdrive attack, a set of Rapid-Fire Fisticuffs, is passed down to Jotaro Kujo and Josuke Higashikata, as their Stands effectively use the same move. You could even say that Jonathan lives on in spirit form through Star Platinum and Crazy Diamond.
Its not a coincidence that every Jojo after Jonathan is a variation of Good Is Not Soft and/or Good Is Not Nice. Jonathan's Nice Guy personality and his father's kindness were squandered on people like Dio Brando, resulting in suffering and death for both; its fitting that his descendants are all hardened in a manner so as to never be abused by people like Dio.
The refutation of Dio's Freudian Excuse has a fair bit more weight behind it coming from Speedwagon, who is implied to have grown up in broadly similar circumstances, but turned out to be a good guy.
Why is Hamon as effective as sunlight against vampires and zombies? The purpose of human sacrifices in ancient Mesoamerican civilizations was to spill the victim's blood, which was believed to be a gift from a sun god. Hamon is controlled by the user's breath and blood circulation.
Speedwagon is the perfect foil to Dio: both are blond-haired street thugs who were antagonists against Jonathan when they first meet. However, when they were shown kindness by Jonathan, Speedwagon chose to use it to better himself and become an honest, successful person, while Dio chose to see it as a weakness and exploited it against Jonathan. To make the analogy stronger, both end up gaining a form of immortality: Dio through his vampirism and Speedwagon through his legacy and foundation, and while Dio's evil deeds continue to haunt the Joestars many generations later even beyond the grave, Speedwagon's foundation continues to help them in various ways even long after Speedwagon's death.
Why does Dio have fangs when he sucks blood through his fingers?
Who says those fangs are for sucking? His body was augmented in other ways, the fangs may just be a side effect.
Redundancy. You can't kill a vampire just by lopping off its head, and if a vampire is reduced to just its head, it doesn't have fingers to suck blood through. We also see that the Pillar Men can consume people through their entire bodies, Dio seems to only be able to use his hands to suck blood, and zombies are stuck using their teeth (where sharpened incisors would be helpful).