Dhampirs (assuming that you are using dhampir from Van Richten's guide to Ravenloft) are quite overpowered, given their racial abilities, which include, but not limited to moving faster than other humanoids, permanent Spider Climb starting from level 3, not needing to breathe, etc.
Adding anything else to the core dhampir abilities is not necessary to make them overpowered.
Changeling's ability 'Shapechanger' is pretty powerful in social-based, investigative or espionage campaign. Though with a bit of creativity it can be applied to dungeon-crawling and combat-oriented campaigns (turn into enemy boss or his captain of the guard or something else like that and walk through dungeon, watching enemy guards saluting you).
Mixing those two would result into overpowered game-breaking character, but mostly because it has Dhampir in it. Compared to Tiefling, Goblin and Variant Human (who are your other players, as far as I understand) - only tiefling may be close to dhampir in terms of 'game-breaking', if he took 'winged' option (because flight allows to escape a lot of problems at low levels and help with lots of things later).
If only 'Shapechander', 'Vampiric Bite', 'Darkvision' and Attribute Increase are included in the race, then it's not as broken, obviously.
Such race is less broken, than a normal Dhampir (though possibility of stacking huge bonus to next attack/check through Bite with some buffing spells/abilities may be worrying). And somewhat more powerful than normal Changeling.
Like Changeling it's powerful at situations Shapechanger can be applied, having unfair advantage over any other races at pretending being someone else.
Vampiric Bite allows to survive better at lower levels and gives potential of getting an extra +10-15 to a check/attack with some buffing and having right feats/class combo.
Downside is that this combo race lacks racial skill proficiency (both original Dhampir and Changeling have 2 skills to choose) - which may limit what character can do outside of combat and usability of 'Shapechanger', making player choose whether they want to pick socially-focused skills of class and background or more universally useful.