I remember back in the day we discovered that some of these trooper skills milestones could only be completed if you were using the vanilla skills, with no star card changing their effects.
I suggest trying this Vanguard Killstreak milestone without using any of the Star Cards that change your Vanguard ability to another Vanguard ability. If you use any Star Cards while trying this, be sure to use only those that don't affect the Vanguard Skill.
Then let me know if this solves your issue. It solved for me back in the day with a milestone regarding the specialist's infiltration skill.
