The Vile Vortices are somewhat ill-defined. I'd suggest you put them on the Tropics of Cancer and Capricorn, because that looks significant.
Given that, Krakatoa is about 17 degrees, or 1,200 miles, north of the Tropic of Capricorn, on the edge of the Wharton Basin. You would need dubious plate tectonics to make anything of that.
However, the Wharton Basin is also where
MH370 seems to have disappeared, which has not escaped the attention of the whole Internet, even though Wikipedia has ignored it. That event could have been what made the PCs' sponsor decide to recruit his field team.