Why else would it exist (if it isn't profitable in either short or long term)?
From Jeff Bezos's 2018 letter to shareholders:
As a company grows, everything needs to scale, including the size of your failed experiments. If the size of your failures isn't growing, you're not going to be inventing at a size that can actually move the needle. Amazon will be experimenting at the right scale for a company of our size if we occasionally have multibillion-dollar failures. Of course, we won't undertake such experiments cavalierly. We will work hard to make them good bets, but not all good bets will ultimately pay out. This kind of large-scale risk taking is part of the service we as a large company can provide to our customers and to society. The good news for shareowners is that a single big winning bet can more than cover the cost of many losers.
Why did the Amazon Fire Phone exist? Why did Amazon Destinations, a local getaway travel agency, exist? Why did Amazon's BuyVIP exist? Why did Amazon Local exist? Why did Amazon Tap exist? Why did Amazon Local Register, a wireless credit card reader, exist? They lost a fortune on those ventures, writing off $170 billion in losses on the Fire Phone alone.
You went on to write:
My only uncertainty is the engineering, maintenance, and hosting costs that amazon incurs.
If you are certain about all of the other costs associated with Vine, you have seen far more information than I have.