Do you know if there is free wifi inside the airport or should i buy simcard and what network that works well in Vietnam?

There is free WiFi at the airport. If you’re going to be traveling all over Vietnam, definitely go with a Viettel sim.

I’ve traveled the country and always had service with them, whereas others with me have used various other providers and been without service for long periods of time. All of them work fine in the main cities though. I wouldn’t recommend getting a SIM in the airport though, as they charge you a lot more for the SIM card, and offer less (and more expensive) plans. You can use the Grab taxi app to get to your friends house, otherwise there is a bus into Hanoi which runs until around midnight.