Just had a chat with the Avios online support - any bonus avios don't count towards the 1500 needed to get the 2000 bonus. So one has to convert a minimum £6.25 worth of CC vouchers to get the 1500 avios, and will then get 1500 + 750 + 2000 = 4250 avios (provided the other conditions of the promotion are met, of course).