How to make a simple USB gadget without vendor and product ids'
Usb is a complicated protocol that does not work the way you want it to work. It must enumerate, must have a VID/PID pair, must report it's structure and endpoints. 

A solution for you would be looking for a usb to gpio or usb to relay board and using that to control your servo (bypass the relay, but timing might not be as good as you expect). Another solution is a usb to serial (ttl) adapter, with the rts and/or dts pins broken out, which can then be toggled by serial port software.

Otherwise you need a usb capable microcontroller solution.