Just to summarise:
On all of the flight options on the LH website, long or short, there was an option to order a Vegan meal. So it has been ordered, and not delivered 5 or 6 times in a row. The issue is that there is an option to order it. Please see the screenshot.
I am not asking for impossible: if it IS available, deliver it. If it is not available, don't offer it. Let us prepare for the trip accordingly. It's not too much to ask for, is it?