Switching to SMF from vbulletin is easy. The database converters are easily available. The only tricky part is to keep the old links alive, which isn’t difficult either with a little regex knowledge and Apache mod_rewrite power.
Since this is a permanent switch, 301 redirects are used to send acknowldgements to web spiders. The old vbulletin installation used SEO friendly URLs and the new SMF has PrettyURLs Mod, which i think despite of no SMF experience at all is a must.
The following is the content from .htaccess file which should reside at the root directory of your forum.
.htacess
RewriteEngine on
RewriteBase /
#Redirect www.mysite.com to mysite.com
RewriteCond %{HTTP_HOST} ^www.mysite.com$ [NC]
RewriteRule ^(.*)$ http://mysite.com/$1 [R=301]
# VB to SMF Redirects
RewriteRule ^forum.php index.php [R=301]
#Rewrite threads, thread id is captured
RewriteRule ^threads/([0-9]+)-.* index.php?topic=$1 [R=301]
#Rewriting archive to wap2 version of pages
RewriteRule ^archive/index.php/t-([0-9]+).* /forums/index.php?topic=$1&wap2 [R=301]
#Rule for direct post permalink, thread id and post id are captured, doesn't seem to work with me
#RewriteRule ^threads/([0-9]+)-.*?p=([0-9]+) index.php?topic=$1.msg$2[R=301,L]
#The following rewrites work only with PrettyURL mod
#Rewriting member profile links
RewriteRule ^members/([0-9]+)-(.*) profile/$2 [R=301]
RewriteRule ^members mlist/ [R=301]
RewriteRule ^members/ mlist/ [R=301]
RewriteRule ^search.php search/ [R=301,QSA]
RewriteRule ^faq.php /forums/help/ [R=301]
RewriteRule ^tags.php /forums/index.php [R=301]
RewriteRule ^sendmessage.php /contact-us [R=301]
#The following requires the activation of RewriteMap http://stackoverflow.com/questions/703709/rewritemap-activation
RewriteMap tolower int:tolower
#Rewrite http://mysite.com/99-Forumname to http://mysite.com/forumname
RewriteRule ^([0-9]+)-(.+) /${tolower:$2} [R=301,L]
#RewriteRule ^forums/([0-9]+)-.* index.php?board=$1 [R=301]