This is a collection of information and theories I have formulated as a result of various in game events as well as lore books.
The Gray Host is a collection of two inner factions, the Gray Council and the army lead by King Styriche. Both the council and army were a collection of various vampiric clans and werewolf packs pulled together in a somewhat corrupted idea of family and friendship.
The Council came first, and acted as the leadership of the Gray Host overall. Their ultimate goal was the complete dominance over all of mortal kind, which comes from a twisted attempt to produce lasting peace. At some point, Styriche began to raise an army for the Council, which involved the recruitment of vampires and werewolves, as well as a dark pact with Molag Bal to further increase the ranks of the army with daedra. This deal resulted in all associated with the Gray Host to be doomed to an afterlife in Coldharbour, including various werewolf members who would otherwise have been Hircine's.
Right, and as stated it is just game mechanics. That being said a lot of vampires in lore share similar powers like vampiric drain, increased strength, and an affinity for illusion magic. Their bloodline is never openly stated but in several quests you see them sending petitions to the other vampire clans and trying to gather their support and members. This allows for a lot of flexibility when giving them powers since you honestly don't know who you're really fighting until they start casting spells.