I don’t know why we’re still on about this, when Vine itself had the perfect solution (in the final months before shutdown)
In the feed, Vines would loop as normal. I think they were still the standard 6 seconds. For longer videos, this acted as the preview. If you wanted to see more, you would tap the video and it would play out in its entirety. As far as I remember it would still loop. I do not remember the max length.
Long videos were hidden behind 6-second looping previews. This system worked really well and it gave us Camp Unplug