Sleeper buses

Hello ! My friend and I are going to Vietnam for two weeks in May. We're students and thinking about using sleeper buses to get around. We might take the night train once for the experience but mainly are trying to stay on budget with the bus. This is where we would need them for :

- Hanoi to Sapa

- Sapa to Ninh Binh

- Ninh Binh to Hue

We were thinking of taking them overnight to save time during the trip. I was wondering if anyone has any advice for booking ? Any companies you recommend? I'm looking on 12Go but there are so many options it's overwhelming.

Thank you so much for your help :)