Nice flavorful pick with the dhampir. That can setup a really interesting backstory as to why they chose this type of monk. The first time I heard of this type of monk was from a Forgotten Realms trilogy of novels called The Haunted Lands. He was a badass character for sure! I'd probably pick either the dedicated weapon with the longsword, or get the unarmed fighter feat from Tasha's and use levels of gloomstalker, and maybe 2 in fighter for action surge for the vampire's known speed, as my multi classes.
This is a more mechanical build. But I grabbed onto the fear aura. I would start as a Dragonborn from fizbans. Green, Black, or Yellow seem thematic for a long death monk. I would go 6 levels of monk, and then detour to ranger for 7 levels. Go fey wanderer. At 7th level if a creature succeeds against your fear aura you can use your reaction to force another creature to make another save or be frightened. Along the way I would grab the Dragon Fear feat, that way you get another fear aura you can use an attack to trigger a fear aura, though I would talk to your dm about letting the dc be based on your monk dc rather than your charisma.
