I think if this is how it works then what's the point of being a vanguard. Landing the final blow on mobs that are being struck seemingly from 3-4 rohirrim simultaneously seems very random. Especially as I can't tell the rohirrim to stop stealing my kills. sounds like vanguard is only useful in side quests where getting kills is a real possibility. I remember though from the beta if I do get at least half way or so in the tree I did get more pips still I only got to use the skills once or twice per EB and don't remember them being a huge game changer. I suspect that vanguard will only be a factor with a nearly full tree.
Welleg , Kelleg, and Gelleg - Crickhollow
WARNING: leveling a warden may cause you to neglect your other characters.