It may sound alright in speech, but in writing it should be, for example, What is very important is how you ... I would not call it ungrammatical, but it's something that does not quite sound right (unidiomatic as velisarius says).
I would not use very since by starting with What is, you are already emphasising important.
An alternative is The important thing is how you ... or, of course, How you treat your subordinates is very important.
I think that something like this would only work if you were already talking in some way about degrees of importance. In the case of your example, I agree with the others that it is not idiomatic. But compare:
How you interact with your boss doesn't matter at all. Very important, on the other hand, is how you treat your subordinates.