What? I wasn't sure how to categorise this, but I did report. If the words "lakini wapi" actually do mean "in vain," it must be idiomatic - which is a lesson set all its own.
I'm not a native speaker, but it seems to be used in a sentence like "Unataka kulala, lakini wapi" – "You want to sleep, but you can't." Or "Nilijaribu kuongea naye, lakini wapi" – "I tried to reason with him, but to no avail."
Up to now in the course 'wapi?' has always meant 'where?'. I cannot see the connection with 'in vain'. Of corse the puzzlement means I will easily remember the phrase and its English meaning.
Google Translate gives 'but where?'. Might this make sense in e.g. 'Ninataka kulala, lakini wapi?'? I.e. I want to sleep, but there is nowhere that I can't. I will ask my East African friend next time I see him.