In this paper, we propose an Aggregated Medium Access Control Protocol Data Unit (A-MPDU) using fragmented MPDUs with a Very High Throughput (VHT) Compressed Block Ack mechanism for use in IEEE 802.11ac Multi-User Multi-Input Multi-Output (MU-MIMO) Wireless Local Area Networks (WLANs). By allowing the use of fragmentation with the A-MPDU, the waste of medium resources in terms of meaningless A-MPDU pads can be eliminated. Using our proposed method, a fragmented MPDU - instead of A-MPDU pads - can be added to fill the length of A-MPDU boundary in Enhanced Distributed Channel Access (EDCA) Transmission Opportunity (TXOP) sharing mode, enhancing multi-user throughput owing to the increased number of frames that can be transmitted within the A-MPDU boundaries. To acknowledge the fragmented MPDUs in the A-MPDU frame, the VHT Compressed Block Ack mechanism is also newly defined. Simulation results show that the proposed method improves multi-user throughput, over 28% at low data rates and by up to 3% at high data rates.