So you have 2 local networks, the one with the VPS and the one with the 5 devices.
You add a file to the vps and don’t want the 5 devices to each download the file from the VPS.
Syncthing can do this, if you connect all 6 devices and share the folder with all. Then each of the 5 devices will download some part from the VPS and share that with the other 4 local devices.
I doubt you will only have 1x bandwidth use, but it should be far away from 5x.