To get jumping to a class definition working in vim when writing ruby, use https://github.com/tmm1/ripper-tags
Out of the box it won't generated nested classes, e.g.
Class which will break if you have lots of identically named classes in individual modules. To fix this you need
--extra=q so the full command is:
ripper-tags -R --exclude=vendor --exclude=chef --extra=q
You can then use
v to enter visual mode after the leading
:: of a definition,
w to move to the end of it and then
] to jump to the definition of a given class.