Can I connect vcc to vdd?
Based on the images used on the manufacturer's page, found two sites WifiPineapple Wiki (very detailed) and Opening The Alfa AP121U (AP121U is the commercial product version of the Hornet-UB board) that use the same image and reference the same board. Both state DO NOT CONNECT THE VDD PIN. So that's the main source of your problems. The WifiPineapple Wiki mentions that any 3.3v uart adaptor will work.

This board is based on the AR9331, an Atheros SoC. Atheros never releases datasheets for the chips, but they have been reversed engineered by OpenWRT users, and these chips are known to use 3.3v.

Oh, and importantly, you want to crossover the uart cable. The Uart Cable TX would connect to the board's RX, and Uart Cable RX connects to the board's TX.