On windows 7 64bit, my installed version of Dub (0.9.20) was having problems fetching vibe-d. I updated and it fetched just fine, but now I get LNK4003 warnings leading into unresolved symbol errors on the fetched packages.
AppData\Roaming\dub\packages\vibe-d-0.7.20\lib\win-i386\eay.lib : warning LNK4003: invalid library format; library ignored
Before this started everything was building fine, including with -m64 in dflags. rdmd and dmd still link fine if I copy the vibe-d parts I'm using into my source tree.
Going back to 0.9.20 did not fix the link errors, so I find it strange that dub was working at all before.