[adamanteus]

Mambas are Elapids (like Cobras) not vipers, and yes, they do lay eggs, but don't break any 'rules'..
Most, but not all vipers are live bearing.. for example Calloselasma rhodastoma is a Crotalid linked closely to the North American Agkistrodon group, but it is an egg layer.... I know this, because I've bred them.