What all does a loxodon lose if he becomes a dhanpir ? Try to see if it's worth it ?

Ancestral Legacy. If you replace a race with this lineage, you can keep the following elements of that race: any skill proficiencies you gained from it and any climbing, flying, or swimming speed you gained from it.

If you don't keep any of those elements or you choose this lineage at character creation, you gain proficency in two skills of your choice.

You keep what it says and the rest is replaced.

Loxodon doesn't give any proficiencies and you don't get any special speeds. So basically it's 2 profs of your choice and everything in the Dhamphir listing and you lose everything else.

Thanks so I lose powerful build and loxodon serenity 

You would lose:

Loxodon Serenity
Natural Armor
Trunk
Keen Smell

You gain 2 proficiencies of choice and the following from Dhamphir:

Darkvision
Deathless Nature
Spider Climb
Vampiric Bite