So Lets Start :
1. First make sure you are running Jelly Bean 4.2.2 Build 15.2.A.1.12. Go to Settings > About to verify this.
2. Enable USB Debugging , Go to Settings > About phone and tap 7 times on Build number to enable USB Debugging mode.
5. Open the vRoot and Connect your phone Xperia M to your PC , you will see a green Root icon in vRoot, click on that to start the rooting process.
6. When it is finished your phone will restart automatically and vRoot will show something like the below screenshot.
7. Congrats your phone is now rooted
Note :- If you want to change the Superuser app to SuperSU , just download SuperSU from Playstore , open it , it will ask for Root Permissions , grant it , then follow the screen instructions to update the binary and remove the other Superuser app , finally restart the application.
Note : This method works on both locked and unlocked bootloader devices!!!