In today's serial communications, there are three 'states' which are 'USB', 'TTL(UART)' and 'RS232C'. The source of the confusion comes from the fact that each maker employs a different method to hook up their TRXs to a PC.
ICOM's CI-V certainly is a nice idea, but it requires a rather proprietary adapter (CT-17). Kenwood/Yaesu fixed TRXs are more handy as they have a DB9 port so can be hooked up with a generic cable (plus a USB-Serial adapter).
It is known that CI-V and FT-817/857/897 cables are easily homebrewed using a USB-UART conversion IC such as FT232R or even utilizing an old mobile phone cable.
As to the GPS sported VX-8G, direct RS232C level signals apparently come out at the data jack. Yaesu sells CT-143 cable for connecting it with a PC, which perhaps is merely a direct wiring to a DB9 connector, not like the CT-62 level converter cable.