I found the solution myself. To anyone interested:

When replacing a DLC vehicle:

  1. Create a folder dlc_<dlc name> in FiveM Application Data\citizen
  2. Open update/x64/data/dlcpacks/<dlc name>/dlc.rpf/x64/levels/gta5/vehicles in OpenIV
  3. If there is no vehicles.rpf file and there is <something>vehicles.rpf file, place replacement files in: FiveM Application Data\citizen\dlc_<dlc name>\x64\levels\gta5\vehicles\<something>vehicles\
  4. If there is vehicles.rpf file, place replacement files in FiveM Application Data\citizen\dlc_<dlc name>\x64\levels\gta5\vehicles\

Example 1:
My DLC name was mpapartment so I had to put the replacement files in:
[...]\FiveM\FiveM.app\citizen\dlc_mpApartment\x64\levels\gta5\vehicles\apartmentvehicles\

Example 2:
Replacing vehicles in patchday2ng update/dlc (OpenIV shows only vehicles.rpf file, no custom folder):
[...]\FiveM\FiveM.app\citizen\dlc_patchday2ng\x64\levels\gta5\vehicles\

Be careful: <something> in <something>vehicles.rpf is not always the same as <dlc name>.

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