[Android Software Development] limited_imagination:
Yes, I think you are on to something here, thank you!

You're right, there is something funny about the .config file.

In my .config file, under the "USB port drivers", there are no entries at all. I just get:

# USB Imaging devices
#
# CONFIG_USB_MDC800 is not set
# CONFIG_USB_MICROTEK is not set
# CONFIG_USB_DWC3 is not set
# CONFIG_USB_CHIPIDEA is not set

#
# USB port drivers
#

# USB Miscellaneous drivers
#
# CONFIG_USB_EMI62 is not set
# CONFIG_USB_EMI26 is not set

As I showed you in the picture above, In the menuconfig program, I can select the options for the various imaging devices and miscellaneous drivers, but nothing appears under port drivers.

So, I tried modifying the .config file like this:

#
# USB port drivers
#
CONFIG_USB_SERIAL=y
CONFIG_USB_SERIAL_FTDI_SIO=y

But nothing changed running menuconfig after that.

Then I took all the driver names listed in /kernel/linux-vespa2/kernel/drivers/usb/serial/Kconfig, and listed them sequentially as they appeared in that file, like this, hoping they would appear in the menu in the same order:

#
# USB port drivers
#

#
# USB Serial Converter support
#
CONFIG_USB_SERIAL=y
# CONFIG_USB_SERIAL_CONSOLE is not set
CONFIG_USB_SERIAL_GENERIC=y
CONFIG_USB_SERIAL_AIRCABLE=m
CONFIG_USB_SERIAL_ARK3116=m
CONFIG_USB_SERIAL_BELKIN=m
CONFIG_USB_SERIAL_CH341=m
CONFIG_USB_SERIAL_WHITEHEAT=m
CONFIG_USB_SERIAL_DIGI_ACCELEPORT=m
CONFIG_USB_SERIAL_CP210X=m
CONFIG_USB_SERIAL_CYPRESS_M8=m
CONFIG_USB_SERIAL_EMPEG=m
CONFIG_USB_SERIAL_FTDI_SIO=m
CONFIG_USB_SERIAL_FUNSOFT=m
CONFIG_USB_SERIAL_VISOR=m
CONFIG_USB_SERIAL_IPAQ=m
CONFIG_USB_SERIAL_IR=m
CONFIG_USB_SERIAL_EDGEPORT=m
CONFIG_USB_SERIAL_EDGEPORT_TI=m
CONFIG_USB_SERIAL_F81232=m
CONFIG_USB_SERIAL_GARMIN=m
CONFIG_USB_SERIAL_IPW=m
CONFIG_USB_SERIAL_IUU=m
# CONFIG_USB_SERIAL_KEYSPAN_PDA is not set
CONFIG_USB_SERIAL_KEYSPAN=m
# CONFIG_USB_SERIAL_KEYSPAN_MPR is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA28 is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA28X is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA28XA is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA28XB is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA19 is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA18X is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA19W is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA19QW is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA19QI is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA49W is not set
# CONFIG_USB_SERIAL_KEYSPAN_USA49WLC is not set
CONFIG_USB_SERIAL_KLSI=m
CONFIG_USB_SERIAL_KOBIL_SCT=m
CONFIG_USB_SERIAL_MCT_U232=m
CONFIG_USB_SERIAL_METRO=m
CONFIG_USB_SERIAL_MOS7720=m
CONFIG_USB_SERIAL_MOS7715_PARAPORT=m
CONFIG_USB_SERIAL_MOS7840=m
CONFIG_USB_SERIAL_MOTOROLA=m
CONFIG_USB_SERIAL_NAVMAN=m
CONFIG_USB_SERIAL_PL2303=m
CONFIG_USB_SERIAL_OTI6858=m
CONFIG_USB_SERIAL_QCAUX=m
CONFIG_USB_SERIAL_QUALCOMM=m
CONFIG_USB_SERIAL_SPCP8X5=m
CONFIG_USB_SERIAL_HP4X=m
CONFIG_USB_SERIAL_SAFE=m
# CONFIG_USB_SERIAL_SAFE_PADDED is not set
CONFIG_USB_SERIAL_SIEMENS_MPI=m
CONFIG_USB_SERIAL_SIERRAWIRELESS=m
CONFIG_USB_SERIAL_SYMBOL=m
CONFIG_USB_SERIAL_TI=m
CONFIG_USB_SERIAL_CYBERJACK=m
CONFIG_USB_SERIAL_XIRCOM=m
CONFIG_USB_SERIAL_WWAN=m
CONFIG_USB_SERIAL_OPTION=m
CONFIG_USB_SERIAL_OMNINET=m
CONFIG_USB_SERIAL_OPTICON=m
CONFIG_USB_SERIAL_VIVOPAY_SERIAL=m
CONFIG_USB_SERIAL_XSENS_MT=m
CONFIG_USB_SERIAL_ZIO=m
CONFIG_USB_SERIAL_WISHBONE=m
CONFIG_USB_SERIAL_ZTE=m
CONFIG_USB_SERIAL_SSU100=m
CONFIG_USB_SERIAL_QT2=m
CONFIG_USB_SERIAL_DEBUG=m

Still no change in menuconfig.

BTW, I had set them all to "m" at first, but got complaints in the terminal after exiting menuconfig, that "symbol value 'm' invalid for USB_SERIAL....."

So it's obviously reading the .config file at that point, but it still doesn't appear in the menu.

Any other ideas? I think you are close to solving it!