I suppose it would be kind of like where to Sternguard squads fit in the normal SM chapters?
Sternguard and Vanguard Vets are from a standard Chapter's First Company, they are then selected to serve as Terminators when required. Considering that the First Company of the Dark Angels is the Deathwing and Company Vets are not from the Deathwing they are just from the Company itself I would assume that they are slightly over the 100 Marine allowance. This is taking into account that they do not list the Company Vets in the Company Composition. I could be wrong how ever and the Vets could be situationally drawn from the squads that make up the company.