I have found a very simple way to rewrite the IMEI on my Lenovo P780.
No backups or extra files needed.
It works for both 8GB and 4GB versions
Rooted phone with any working ROM installed.
You can install stock rom from stympy's thread in P780 General section.
You can use Framaroot to root the phone under Boromir method if on stock rom.
MobileUncle MTK Tools installed on ROM
Download the attached zip file and extract the file.
Enter IMEI for SIM 1 and then enter it for SIM 2 when asked.
A IMEI.bak file will be created in the same folder where you placed the imei software.
Now copy the IMEI.bak to your sd card or phone.
Open mobile uncle tools and select IMEI backup and restore
select and restore it from sd card.
Disclaimer : I do not own any content in this. I found it all on google or xda. I do not remember which