Parked, not moving
It's worth saying plainly: a QR code on a moving vehicle, viewed by someone in another moving vehicle, is not a scannable scenario — the physics of a phone camera focusing and a code holding still just don't line up. The realistic use case is a van parked at a job site, a truck stopped curbside for a delivery, or a car sitting in a lot.
Once that's the assumption, the placement question gets a lot simpler: design for someone standing a few feet away from a stationary vehicle, phone out, with a moment to line up the shot. That's a very different design brief than a billboard meant to be read at speed.
Size and placement for a parked vehicle
Put the code near eye level on a side panel or the rear door — spots someone naturally looks at while walking up to or past a parked vehicle. Size it generously; curbside viewing distance is still a few feet, not inches, so think in the 20-30cm range at minimum, not a business-card-sized sticker.
Strong contrast against the wrap's base color matters more here than on paper, since vehicles sit in direct sunlight, shade, and everything in between throughout the day. A dark code on a light wrap panel, or vice versa, holds up far better across those lighting swings than a low-contrast combination.
Flat panels beat curved ones
Vehicle body panels aren't flat the way paper or a sticker sheet is — wheel wells, rounded corners and heavily sculpted panels bend the code's modules the same way curved product packaging does, and that distortion can be enough to break a scan. Doors and rear panels tend to be the flattest real estate on most vans and trucks, which makes them the more reliable spot.
If a wrap design calls for the code to wrap around a curve anyway, keep it as small a portion of that curve as possible and test a printed proof against a few different phones before committing to the full wrap run.