The important things is to use every SPAM option that is available! I have two forums up at
AntiqueRadio.NET and
Transistorized.COM. Both are completely unused despite a fair bit of 'curiosity' traffic. Neither has any SPAM though to the best of my knowledge. I did put up a couple of other forums without the SPAM modules. Both became clogged with hundreds of junk (and much worse) posts. The good news is that the SPAM protection, at least for SMF, works well! I would also say that SMF seems to have more updates that phpBB must if 2-3 years is accurate. SMF seems to have a lot more graphic control and many more base 'skins' than phpBB seems to have.
Again, it is highly unlikely that people will flock to your forum. If you are rich, go ahead and buy vB. No one is arguing that it is a good system. If you aren't rich (or are and want to stay that way), go for one of these decent free forums at least until your forum is used significantly.
For reference, the phpBB forum that I mentioned previously is at
AntiqueRadios.com. It is a high volume forum that seems to work quite well, to be fair. Like DNF, they have been up for many years. I doubt that any new forum will put a noticeable dent in either of these forums which are well established for their respective communities.