Unfortunately the only way to get type errors statically right now is to run `hh_client` at the command line (see http://docs.hhvm.com/manual/en/install.hack.bootstrapping.ph...) -- this is what vim-hack will do for you. HHVM will only report type errors that it can see at runtime, which won't catch the edge cases or error cases you didn't think to test, which kind of defeats the purpose of having the static type system. For a really simple integration, you can do `watch hh_client` in a second window, which will run the checker every 3 seconds, configurable, see `man watch`.

This separation is different than many other languages, and lots of folks have found it confusing. It largely exists for technical reasons, and we should be able to have a better UX for end users; this is something I've been thinking a lot about and want to try to improve in the language going forward. (I work full-time on the Hack team.) Please do give it a shot and let us know how it goes!