Good article. Thanks.
I always thought doing something in vain meant you did it in an unserious way. If i open a savings account in vain, then I’m not serious about saving.
If Trump said “those GD democrats” he could well have meant it. And they probably are.
Those who do evil in the name of the Lord would fit this definition.
You have wearied the Lord with your words. But you say, "How have we wearied him?" By saying, "Everyone who does evil is good in the sight of the Lord, and he delights in them." Or by asking, "Where is the God of justice?" Malachi 2:17 ESV
It could also mean you did something without results, or without hope of results. Like "I tried to catch the fly ball as it went over the wall, but it was in vain".