Fields in the packet beginning with VHT are new to 11ac. (VHT = 11ac and stands for Very High Throughput.) The VHT-SIG-A field contains two OFDM symbols.
VHT-SIG-A: Contains info required to interpret VHT packets (BW, number of streams, guard interval, coding, MCS, beamforming)
The first symbol is modulated using BPSK, so any 11n device listening will think that the packet is an 11a.
The second symbol uses a 90 degree rotated BPSK which allows the VHT device to know that this is an 11ac packet. Important information is contained in the bits of these two symbols such as bandwidth mode, MCS (Modulation and Coding Scheme) for the single user case, number of space time streams, etc.
The legacy fields and the VHT-SIG-A fields are duplicated over each 20 MHz of the bandwidth and the appropriate phase rotation is applied.