When trying to install vagrant for the second time, I ran into an error about installing the nokogiri gem again. (This happened to me before, but I since forgot how I fixed it). Thank god nowadays there is stackoverflow, so a solution to most problems is never far away. This post describes that one only needs to export an environment variable when trying to install the vagrant_fusion plugin.
NOKOGIRI_USE_SYSTEM_LIBRARIES=1 vagrant plugin install vagrant-vmware-fusion
I hope I remember I have posted this the next time I upgrade my mac.