I think the problem is:
1) Even though the Countess doesn't know that the MC is an Ashall, she does know that Lillian allowed him to negotiate on her behalf. I suppose she could interpret it as a deliberate snub by Lillian and get in a huff over that, but still, the MC clearly has high powered backing.
2) Its wasn't clear what the Countess wanted to achieve at the meeting. All she really did was present a declaration of war. The Ashalls seem to have been top dogs for generations, they're hardly going to back down without some demonstration of Rimsin's power.
3) If the the Vampires really believe Rimsin's unstoppable and wanted to gloat then that might make sense... but shortly afterwards he backs down from a fight with just one unknown sorcerer. Presumably he's still regaining his power. Why warn your enemies you're coming for them when you're not ready yet?
There's nothing that can't be explained by inate arrogance and a failure to read the Evil Overlord List, but yeah, the Countess didn't come off as much of a threat.