How to answer a non-specific point of a review?
I guess I won't be too popular for saying this, but you seem to have the wrong idea about what is going on. The point isn't to "defend your paper" or "rebut the reviewer" but to take constructive criticism of the paper so that you can improve it for the conference and for the future. It isn't a debate. It is a search for a way to express some scientific truths through a cooperative process. 

That said, you can't respond to what you don't know. As user Miguel says in a comment, the editor may be able to help you here if you say that you don't understand the comment and ask if the reviewer can give more specific advice. 

When you get the advice, hopefully you will, you can take it into account for a revision. But not just to complain about it. You won't win much by arguing. Your paper will only be accepted if it stands on its own, without side arguments in its favor. 

Please note that I'm not being insulting here. Nor were your reviewers if they made suggestions about the paper that you didn't really want to read.