VBR can be hit/miss.. depends on how well the camera handles it, but I find it lowers the bitrate too far for my tastes.. at least on recordings.. if your short on disk space its surely something you should consider.
I run VBR on my substreams, really helps when your on a mobile data connection.. but I record w/CBR, because I have more storage than I really need.. except my LPR camera, since its basically a black image @ night, its being recorded as VBR and its a huge improvement in storage required.