Unfortunately it looks like that mod is locked behind their Patreon, so I can't easily check compatibility.
I'm fairly sure that SonozakiSisters' Deadly Vampires won't make the autonomous NPC vampire attacks from my A Reason to Slay mod deadly - I had to create custom interactions for the autonomous vampire attacks, in order to test for the different outcomes, and unless SonozakiSisters did their mod in a very specific way (or deliberately made their mod compatible with mine) the deadly bite function won't happen because it'd be somewhere else in the coding.
I don't think the two mods would conflict, though - so you could have A Reason to Slay running for NPC vampires and have your played vampires be deadly with SonozakiSisters' mod. You might need to delete the included crosspack module, though, depending on exactly how they did their coding.