AUC students are fully registered students at both the VU and UvA, but you only pay your AUC tuition fee once, and to just one university: either UvA or VU. If you pay your AUC tuition fee to the UvA, you are "UvA-first". All international students (non-Dutch students) are UvA-first. If you pay your AUC tuition fee to the VU, you are "VU-first".
AUC does not handle your university enrolments and does not process your tuition fee payments. The central student administrations of the parent universities take care of all payments. Please follow the payment instructions as given by the universities.