adding a column to the user table
 

straightforward in mysql or do I have to somehow register the new field within vb too?

You will only need to add the field as vBulletin will grab the whole row (SELECT *).

Wouldn't it be easier just to add a custom userfield (private if need be)? or is there some advantage of adding it on the user table?

It depends on the purpose of what you are trying to achieve.

super - thanks.

Basically I want to use the field to store which banner was last viewed so I don't repeat them. I can store banners and any additional info elsewhere if I need to (not that bothered tbh.)

I'm not sure about custom userfields to be perfectly honest but I'll have a look around.

Cheers

Custom userfields are only really needed when you want Admins/Users to edit a database field, in which this case there isn't a need.