Ensure you own the van and then in a helicopter: (I think Orca and Taru can lift a van).
1) Press Right CTRL + B to open the sling load assistant.
2) Hover above the van at below 10m and you should see a small diamond appear on your sling assistant.
3) Press B and align the lines on the assistant and hold for a few seconds and the van will be hooked.
Watch this if still struggling: In the video, he sling loads a crate but it is still the same procedure.