[wolfbln]

Hi to Vietnam.
From 15 June 2017 the EU will adopt the Roam like at home principle. This means you can buy a SIM card in the first EU country and use it in another EU country at the domestic rate.

Note, that Switzerland is not a member of the EU and steep surcharges still apply. You should switch off data there or buy a separate SIM for Switzerland as only few SIM cards in Germany give good rates for Switzerland.

In summer the registration rules for prepaid in Germany will change for online registration. So it's better to a go to a store of the networks (Telekom, Vodafone, O2) where they can register you showing your passport and giving a local address (like your hotel) than to buy it in a supermarket or petrol station where you can't register.

In Germany only few SIM cards are sold in airports. You will have free WiFi at the airport. Better wait to go downtown to Munich center, where you can find many places to buy a SIM card. But have your passport with you and do the registration at the point of purchase.