On the SMF side:
1. Export your SMF 1.1.15 database.
2. Install a new full SMF 2.0.1 forum on a new empty database.
3. When done with the installation, log into phpMyAdmin and drop all the tables in the database that the installation has created.
4. Import the 1.1.15 database you exported in step 1.
5. If you have installed 2.0.1 in a new path on the server, edit the paths in the smf_themes table to correspond with the new path!
6. Make sure the database prefix in Settings.php (in the root folder) corresponds with the prefix of your imported database.
7. Download the Large Upgrade package for SMF 2.0.1 and copy upgrade.php and all .sql files from it to the root folder.
8. Run upgrade.php in a browser and wait for it to finish (ignore the SQL errors when done).
On the Xenforo side:
1. Install a new full version of Xenforo 1.1.0 beta 5.
2. Download and install the unofficial SMF 2 Importer.
3. Edit /library/XenForo/Importer/SMF2.php and remove:
$import['identities'] = array(); if (!empty($user['icq'])) $import['identities']['icq'] = $user['icq']; if (!empty($user['aim'])) $import['identities']['aim'] = $user['aim']; if (!empty($user['yim'])) $import['identities']['yahoo'] = $user['yim']; if (!empty($user['msn'])) $import['identities']['msn'] = $user['msn'];
5. Choose SMF 2.0 and fill in all the database information where your converted SMF 2.0.1 database is residing.
6. If you get some errors, click Import External Data again in the breadcrumb and choose continue.
7. Import User groups, Forums, Users and the threads/posts (the last import takes a long time!).
This have at least worked for me, but if you try this, make sure you have everything backed up before doing something critical to your database! Good luck!