Here you can download USB and ADB driver for your Lenovo P780 smartphone by Lenovo comes with Mediatek MT6589 cpu. This driver will also work for USB ADB and Vcom Preloader Usb for Flashing handset. Just download from below link and run it. Remember this driver for only PC Windows.
Download Lenovo P780 MT6589 VCOM CDC MODEM Preloader USB ADB Drivers Official
Supported OS : Windows XP / Windows Vista / Windows 7 / Windows 8 / Windows 8.1 / Windows 10 both 32 and 64 bit