Note that sometimes this error can be caused because there are two VDF files provided by metamod - one for 64 and one for 32bit. If the server says that it's failed to load metamod, it may well just be talking about loading the 32bit metamod and had in fact successfully loaded the 64bit one but didn't tell you.
Before panicking (like I did) and trying to find why metamod isn't loading, run
to check whether it has in fact successfully loaded