There is a new ROM for Lenovo S660 using the ROM for VK2015 (www.vkworld.cc) surprisingly compatible with S660.
Download it from: https://cloud.mail.ru/public/N2i2/f5G8tjqBs.
The ROM is not rooted but you can easily root it using iRoot via PC, dowload it from http://irootandroid.com/. There is also apk for it, but did not try it. the one for PC (Windows) worked for me.
How to flash?
Disclaimer:
You are doing this at your own risk, I am not responsible for any damage to your S660 or things that may happen when you are flashing it.
You must setup up connection of usb from your computer and S660 (use MTP Mode and USB debugging enabled!) and SPFlash Tool (find alsewhere to do this, ):
1. after downloading the ROM zip file, extract it to any location of your choice, an remember it
2. Run SPFT and click scatter loading, when asked point to the directory location of previously extracted ROM, and choose the scatter file shown.
3. Turn-off your S660, and remove the battery.
4. In the SPFT windows, choose Firmware Upgrade+download from the dropdwon menu (just below the scatter file), and click Download from the Menu (Download and Stop)
5. connect your S660 (battery removed) to the any USB port on your compy, if you have set up connection between S660 and your computer, just wait for red bar to appear, followed by purple, and finally yellow that will take sometme to finish.