MyBB

for the special characters like â€
can you check character encoding used for posts & database collation in vBulletin ..

Reg. BBcodes appearing like [INDENT]
for the vBulletin BBcodes try to add equalent MyBB MyCodes
or use SQL queries to remove those non-existing BBcodes

eg. SQL query

UPDATE `mybb_posts` SET `message` = REPLACE (`message`, '[INDENT]', '') WHERE `message` LIKE '%[INDENT]%';

characters like †in posts can be also removed by using SQL query like above ..

[common sql queries guidance]