How to make sure that the data is delivered to the server?

Connection breaks are possible.
How can the hardware device verify the data has been delivered to the server?
How to implement feedback?

You could simply read back the vPin data and compare it… lots of unnecessary code, but doable.

Besides, if the device is properly coded, it knows if it is connected to the server or not, and can fall back to whatever logic you wish it to.

See here for example - C++ Blynk - Code Examples for Basic Tasks (Work in Progress)