have just returned from my 2 week trip from Johannesburg to Beira & back & have only been stopped once at a roadblock (in Maputo) & that was to check my driving licence.
I did check with the south african AA before I left regarding the triangle/vest issue but they informed me of yet more interesting comments such as if there are more than 2 people in the car you need to have 2 reflective vests !
In the end I bought 1 reflective vest at macro before I left SA & took my old triangles with me. The reflective vest seems to be a definite. They are on sale at the Komatiepoort border crossing & at a lot of other places in Moazambique along the road. The triangle issue is unclear but when I asked the mozambican guy at the Hollard 3rd party insurance kiosk at the border he said that you needed a vest & 2 triangles.
In the end it wasn't an issue. Interestingly at the 1st toll booth after entering mozambique cars with foreign plates are receiving free mozambican vodacom sim cards. In this pack there is a small information leaflet in english /portuguese which mentions driving rules. In this it is mentioned you need a reflective vest & 2 triangles (size & type not specified). I kept this handy just in case .....