Is it possible to control the strength of the vibration of the phone when ringing?
No. Android doesn't give apps a way to control the strength of the vibration, so you won't find an app that can do this for you.

That said, there's an app that claims to be able to change the vibration strength on Nexus 4 and Nexus 5 phones, using a device-specific method that requires root. Some users report that it doesn't work at all, but if it does work and you have a supported device, and your device is rooted, you may be able to find an app that does what you want.