I'm assuming yours is a simple set up, as there are options that have map lights, and ambient lights. I haven't a clue how the 2023 Vito is configured.
As said wire to the existing light.
The switched feed will be from the SAM, don't be looking for door switches they're gonna be within the lock mechanism.
The -ve will be brown.
There will be a permanent +ve
t'other should be the switched +ve, but I have seen the -ve as switched.
On mine the rear lights will time out after 20 minutes, so if I want longer I need close / open a door again, but that then only provides another few minutes.
There are LED units available that would pick up all 3 wires from the existing. Then you would have choice to just switch it on when needed.