In the old version (Iridium) there was a method Vector.SquaredNorm() but in the most recent stable version of Math.Net there is none available.
What method should I use?
If you want the squared L2-norm (which is what Iridium did if I remember correctly) you can simply square it yourself:
var squaredNorm1 = Math.Pow(v.L2Norm(),2);
Alternatively you can also use dot product which is a bit shorter (and also faster in case you use native providers and the vectors are very large):
var squaredNorm2 = v*v;