Oli,
I'd suggest to address this question to GPIB gurus, speaking in natinst.public.gpib.general or on exchange.ni.com,category GPIB.
From my knowledge of this subject, you should start VISA session as a listener on GPIB bus.
Hope that VISA event vi's will do. Or may be E.g. Wait for request vi will be listen the bus. Then you'll be capable of establishing right connection and only then will be able to get data from instrument. This is only my general understanding, not a solution itself.
Good luck,
Sergey