Well, then it's a simpler joke. I recall there being a SDM bean throwing article, but forgot the details - I'm the one who's sloppy. In that case, game2011's speculation is entirely correct.
To interpret it through lens of the current canon's metaphysics, I think because Gensokyo is a land in whose language vampires are connected to oni, and unlike the outside world, without TV and movies, Gensokyoans are not very familiar with the concept of Western vampires (it's plausible that they did have heard of this concept through books and such), they would intrisically and subconsciously believe there is a connection between them, and this belief would force some oni traits onto vampires.
It's very likely vampires would not have those oni weaknesses outside of Gensokyo. In that very article, Patchouli gave us an explanation to this roasted beans weakness, connecting it to Touhou oni's backstory. If what she said is true, it would make no sense for vampires to share oni's weakness, as they have nothing to do with this backstory - unless the weakness is somehow passed from oni onto vampires.
Judging from the article and IaMP Patchouli ending, SDM probably had never made any roasted beans before, so they wouldn't even know whether roasted beans hurt vampires.
EDIT:
I've been reminded that the real life explanation for roasting beans' power is that the word for beans is pronounced "mame", same as 魔滅 "destroy demons" (of course that's just an explanation; the actual belief that beans can ward off evil originated in China, long before the Japanese invented the youkai "oni"). So in a Japanese context, throwing beans are supposed to drive away not only oni, but all evil - or you can say here oni is a stand-in for all evil. ( Japanese Wikipedia:
here.) It's quite reasonable that in Gensokyo, beans can also harm demons (in Touhou vampires are demons). Patchouli being a Westerner, cannot see this connection, and invented a convoluted explanation. The writing technique employed here is similar to CoLA, where Rinnosuke's pseudo-intellectual ramblings often conceal the truth of the story.
That leaves us with sardine head and holly branch... Their explanations will probably be something very similar.
