Do you think.. that 250mb diskspace will be enough for a forum PHPBB3 with over 5k members, 400k post and over 200k threads?
I highly doubt it. If your hosting provider is only offering that much space to you, I would suggest finding a new one!
In one simple word - no. On average, mysql is going to take up 100 megs, per 100,000 post. With 400,000 post, your going to need at least 400 megs of storage. Thats if your host counts mysql towards your total disk size.
That doesn't make any sense. It seems like you're trying to say that because you're using vbullitin you have those things when in reality vbullitin has absolutely nothing to do with those things. It's something your host controls, not your software.