multi entry visit. how can I solve in order to avoid anoailies in terms o visa policy?

The easiest way is to apply for a multiple entry one from the Office of the Commissioner of the Ministry of Foreign Affairs in the Hong Kong SAR. However, I am afraid that they may not issue a multiple one to you. You may be granted a single entry one for your Beijing Trip.

I'd like to know how long you will transit in Guangzhou before you fly to Ho Chi Minh. If it is less than 72 hours, you can make use of 72 hours free transit (HK-Guangzhou-Ho Chi Minh). The premise is that you must have valid Italian passport and air tickets in hands.

For your return trip from Ho Chi Minh to Guangzhou and then HK, you actually can also make use of 72 hours free transit too if you stay less than 72 hours in Guangzhou. However, you have to make sure that you fly from Guangzhou to HK. Unfortunately, you take train back to HK. In this way, you need to apply for another entry permit for Guangzhou. Your choice is to apply for a L type from Chinese embassy in Vietnam.