Even if you could change it to
Code:
[attachment]tigeraugusta.jpg[/attachment]
That isn't going to link to anything. In fact, that isn't even valid BBCODE in vbulletin. vbulletin would use [ATTACH], not [ATTACHMENT] and it would use a number that identifies a specific attachment in the database table. So really, if that is what you have in the posts, you will want to remove the whole thing, not change it.