I've just started a new game on the 360 with dawnguard installed from the get go.

My PC is level 8 and in dragonsreach, when I leave to whiterun two master vampires and three death hounds rush from the main gate and destroy everyone in their path. With the help of guards and npcs I can kill them easily enough but npcs usually die in the process.

I have no saves for hours before I went into dragonsreach.

My question is, if I turn my difficulty up to master will the npcs and guards stand a better chance of killing the vampires with minimum difficulty?

No. Difficulty setting only effects how much damage you do and how much damage you take. It has no effect on any other NPC including your followers.