I ran into the same issue, but I was able to solve it using the Universal ADB driver*. The driver can be found at: https://github.com/koush/UniversalAdbDriver
*NOTE: I had to android_winusb.inf to include the ViewSonic device info:
;ViewSonic
%ViewSonic% = USB_Install, USB\VID_0408&PID_3998&REV_0216&MI_01
%ViewSonic% = USB_Install, USB\VID_0408&PID_3998&MI_01
The updates have been merged into the main branch already and should have to is clone the repo and install the driver manually from usb_driver.