JoJos are not vampiric; I specifically mentioned that extensive human-focused breeding would have removed almost all vampire heritage from the bloodline, thus rendering them more-or-less a family of humans who have a weird, ridiculously-inheritable birthmark. I mean, most R+V monsters don't seem to have mutagenic DNA (which would be required for traits to be inheritable for more than 200 years without external contributions- or, to be more explicit, without more of the ancestor's species breeding with one of the descendants), so extensive dilution to the point that they are, for all practical purposes, purebred members of the other species is, indeed, possible.
Basically, if about 99% of their DNA is human, then they'd most likely be able to use Ripple without ill effect... and frankly, probability dictates that MORE than 99% of JoJo DNA was human by the time JJBA started.
Look at it this way: Can you actually say for certain that the first Ripple JoJo wasn't Jonathan? That'd make it... like, 350-ish years of diluting the bloodline (depending on when the proto-Joestar marriage occurred), thus rendering him basically safe, since that'd be a minimum of about 15 generations between him and Akasha if this guess is accurate- I mean, that's 1/32,768 (yes, just over one thirty-three THOUSANDTH) at 15 generations, and it'd just get more diluted as time passed; the vampiric heritage would probably be pretty much negligible within 8 generations, and it may actually be completely GONE on a genetic level (except for the apparently-mutagenic birthmark) by the time Jonathan was born.
In short: Your assertion that the first Ripple JoJo would have burst into flames at his first attempt to use Ripple basically assumes that the child of a human and a vampire will always be 100% vampire, without exception. Simple math, on the other hand, indicates that, at least by the time Jonathan (the first of three known Ripple JoJos) was born, there was basically NO vampiric DNA left.
...This post kinda got away from me.
