It looks like this was due to a mistake in the order certain actions were performed:
1. Convert the vBulletin BB-Code to XenForo BB-Code.
2. Before testing for active tags, remove content appearing between NOPARSE, CODE, PHP, HTML, COMMENT tags.
3. Search for remaining REDIRECT tags.
When importing from vBulletin into XenForo, NOPARSE is converted to PLAIN. This resulted in #2 failing to detect NOPARSE, because content between PLAIN was not removed (VW3 content containing PLAIN is not expected, since the source for VW3 is always vBulletin).
I have fixed this in the next release by swapping 1 and 2. The conversion is still necessary because of the form [REDIRECT][NAMESPACE-BBCODE]Article[/NAMESPACE-BBCODE][/REDIRECT] which was allowed in VW3 as an alternative to using the Prefixed:Article form. To avoid BB-Code name collisions, it is possible that NAMESPACE-BBCODE was imported to XenForo with a different name (like if you named a VW3 tag 'MEDIA').