The solution is... we need not
MediaTek USB VCOM (Android) (USB\VID_0BB4&PID_0005&MI_02), but
PreLoader USB VCOM Port (USB\Vid_0e8d&Pid_2000) !
I have MTK6582 (KingSing S2), the host - WinXPx64.
It looks like my phone (and, maybe, other MTK devices too) have several engineering modes.
I have spent several hours trying to get working
VID_0BB4&PID_0005&MI_02, which is exposed, if device is turned on while holding VOL- pressed. In this mode some strange factory mode with nonfunctional menu items is displayed on the screen. But to activate mode with
Vid_0e8d&Pid_2000 some trickery is necessary.
Sometimes this USB interface starts repeatedly to appear and disappear every half-second in infinitive loop. I had such situation once, when connected turned off phone to USB cable
with battery removed.
I just literally catched
MT65xx Preloader by double clicking on it in windows device manager and pressing "install drivers" in it's properties. I have used
Driver_Auto_Installer_v1.1236.00\SmartPhoneDriver\x64\Infs\usbvcom.inf from
http://spflashtool.com/download/Driver_Auto_Installer_v1.1236.00.zip.
DriverVer = 12/24/2011, 2.0000.0.0
After clicking "Read Back" in Flash Tool this insane loop stopped and program started to function as intended.
This interface could also appear once and for very short moment, if turned off phone (with battery inserted) is just connected to host. Also it could be seen again, if power button is pressed. If "Download" or "Read Back" is not pressed in advance in Flash Tool, than boot will continue after smaller than a second pause (if device is functional).
I don't know if running
Install.bat (from archive I have given above) is necessary. It didn't installed driver when
Vid_0e8d&Pid_2000 wasn't active, but, I guess, running it during short blinks would give the same effect.
Also, I don't know, if advice given in comment
#6 in this thread four years ago would work. Maybe, the fault was in older version of driver (DriverVer=05/30/2011,1.1123.0).
Another reason could be that necessary USB interface disappears before windows manages to install driver for it for the first time. Possibly, only if "catched" in device manager, user could manually assign driver to it.