No, it's not on the list. Probably because it is an oily fish. But I was on another vlcd, W8, and I think they allowed it. It is protein, and the fish oils are healthy ones. I'd say have it instead of tuna. No I don't think it will greatly affect your weight loss.
It won't kick you out of ketosis but does have more cals in. As a VLCD is a very low calorie diet, if you start upping the cals then you obviously risk slowing your losses. But you'll stay in ketosis and will continue fat-burning, so it's swings and roundabouts really. It's got to be your call although the official CD line is "no", and it's best to stick to the plan 100% if you can!