XF 1.5 vB to XF1.5 import, custom user fields fail

[MGoT_AC]

I'm trying to import custom user fields from vB 3.5 into XF1.5, but hitting an error:
Mysqli prepare error: Unknown column 'fieldname' in 'where clause'
Digging further, the sql statement behind this is:
SELECT varname, text FROM phrase WHERE languageid = 0 AND fieldname = ?
and it's correct - there is no fieldname column in vB.phrase

AFAIK, there's nothing special about the vB user profile fields so I don't see why it should fail. (There is a fieldname in vB.phrasetype but that's not what it's looking for)

I see there is a post (https://xenforo.com/community/threads/dont-know-where-to-start.24937/post-303098) saying "You will need to customise the importer for the custom field you have in the table", which I haven't done - if this is the step I'm missing, can you point me in the direction of guides for this, please?

Thanks!​

 

[MGoT_AC]

Alternatively, can I:
1. skip the 'import custom fields',
2. preset the required custom fields so that the table entries appear in xf_user_field_value when I import the users then,
3. fill the table via MySQL ?