I have successfully (after 2 days of tinkering) imported my old vbulletin forum to smf. I am new to smf so still finding my way around it
I had loads of errors until I finally settled on the following
here is what I did (hope it helps someone)
1) convert attachments in vb forum from the file system to the database
2) set php version using htaccess to php5.2 (later versions caused errors, older versions not supported by my host)
3) install a clean installation of smf 2.0 RC3 (couldn't get newer versions to work without installation errors)
4) use the vb4.0 converter
5) attachments were not initially showing until I performed maintenance tasks in the new installation (not 100% sure which one did it, but one of them did)
Once all is installed you can remove/edit htaccess and use newer php versions
Very please so far, good luck!