You can try to check how much Serverload is on the new host. If you are on shared/reseller hosting account then it possibility someone on the same server is using more resources making you site lagg. You can check server load from cPanel > Server Status or from vBulletin AdminCP Index page.
You use the outbound link checker see which other site/link you vBulletin is calling
http://www.linkvendor.com/seo-tools/outbound-links.html
Sincerely,
Tahir
PS:- You can post or PM you forum URL i can try to check if any vBulletin links are calling your dyndns server or not.