Only booking confirmation, not tickets

Federico, since you are Italian citizen, you do not have to bother about this visa free transit and other formalities. Italian citizens can enter and transit China absolutely visa free in the same way as if you are traveling to UK. You will just fill out arrival/departure card that you will receive on the plane from Hanoi to Guangzhou and upon arrival just stand in the queue for exit immigration in order to catch you next flight to Chongqing.

Beside this, even if you were not eligible to enter China visa free, your tickets (booking confirmations) would have been enough and boarding passes are not a crucial detail. Chinese authorities would be perfectly aware that you will make a check-in upon landing in Guangzhou for your continuing flight toward Chongqing.