Ignoring the fact that you could Always get Lucky and have him fail all his save-rolls and such.
A vampire lord on dragon has 14 wounds.
With a 3+ save that'l require an effective 42 attacks
With an additional 5+ death save that ups to 63
6 rippers will on average (taking into account the -1 to hit and swooping dive and assuming you're near a toad) deal:
7 spear attacks at 5+/4+/-/1 averaging: 2.91 succesfull attacks
18 Claws at 4+/3+/-/1 averaging: 12 succesfull attacks
18 beaks at 5+/3+/-/1 averaging: 17,778 succesfull attacks
meaning that on average you'l be doing ~31 succesfull attacks which is about half what you need.
So basicly, your rippers don't stand a chance in hell without support unless you get ridiculously Lucky.
As others have pointed out, you're probably better off getting his vampire stuck on wave upon wave of skinks while your rippers go around murdering his other units. His non-heroes seems squishy enough that you can take them out with relative ease if you focus fire, so getting a hold of the objectives should be relativly straightforward in most situations as you should be able to win when it comes to sheer numbers on the field.
