I used the chaos knights for my blood knights and they worked excellent... switched their heads for ghoul heads, and it looks pretty good if I do say so myself... plus here in Canada Blood Knights are $110 a box while the Chaos knights are $25... so it was a no-brainer for me...
As for weaknesses, I think one of the greatest weaknesses (although some others would see it as a strength) is the reliance upon bunkers and ION

. Just sitting there raising huge tarpits around you and playing point denial can be boring for opponents to play (and the vamp player as well).... plus I believe it doesn't encourage tactics and risk taking, as we become so scared about our general biting the dust (rather than biting our opponents!) that we hide them, or miss out on tactial moves out of fear that they'll be destroyed... not saying all who play a bunker don't use tactical moves - not that at all... it's just that it makes it easier for new vamp players to rely on solely that tactic, rather than expand their skills....
An aggressive vamp list gives more opportunities for opponents to fight our vamps head on, which seems to be a little more fun for them.... until they see what happens when you get them into combat that is!
Another weakness is combined charges... makes mincemeat out of our ranked troops, and I learned the hard way early on what happens when you tarpit with zombies and bring a heavy hitter in to

... they just attack zombies and amass HUGE Combat Res against both units!