How long does it take?

Hello. I have about 4k members, 400 active, only 274 threads thus far.

My site (diablo3 dotorg) is about a game that hasn't released just yet. We expect it to release in a month or 2.

My question is, how long does it take for VBSeo affects to kick in and to see good rankings and more thread posters?

Thanks

Starting is tough, even with vbseo. vBSEO can only help rank content that you have. If you don't have the content, there's nothing to rank you for to get traffic. Instead of focusing on waiting for vbseo to 'work', you should instead put forth efforts to generate more and more content.
All i can suggest is for you to post post post and post some more! Get everything up there that you can.