They arent an exception at all. This is either artistic liscence or the artist is just saying that Remilia is in powered up mode.
Patchouli herself used a Rain spell to keep Flandre in the mansion. If rain didnt have an effect on her, Flandre would have just busted out. So weakness to rain is diffenetly a canon as far as ZUN's vampires go.
I only give a small nod to Remilia since its possible Remilia could have learned how to either push through Rain or maybe she learned how to not totally crumble in it.