The best thing I can really think of doing... Would be to calculate the swing speed of a weapon, given character attributes (dex/stam) and weapon attributes (speed/ssi), in order to determine how often the character would swing, and to then attempt to "blindly" log hits and misses with a journal scan. IE: The script calculates that there should be a hit right now, so it watches for "your attack pierces" or whatever the msg is. if it doesn't see it, then it flags it as a miss..
Another way to do it would be to have it compare the pixel, when the ability is used, it checks the journal if != "attack msg" then it flags it as a miss... The only thing I dislike about this is that you're gonna get someone who knows about this weakness, who runs up on the sampire and yells "YOUR ATTACK PIERCES--" whatever the msg is, next thing you know you have a sampire that's gone full on retard. (Wouldn't be an issue with me though, I change the messages when utilizing journal scans)