Others may have better advice, but I treat it as a process of trial and error.
I usually begin by picking out the main verb, because that gives me the best sense of what's happening in the sentence.
Try to decide if it's a transitive verb or not. You have to understand what sort of action the verb describes.
The verb is "to vent." It's transitive, so it takes a direct object. In this particular case the object is compound.
"To vent" means to release something, or to allow something to blow off. That something would be the object.
What is the something they're releasing? Their bad feelings, specifically, fears, frustrations, and anger.
If instead of three objects there were only one,
anger, the sentence could be ambiguous, because we can have anger
at someone.
So we might be talking about "
their anger at the bankers and the politicians, and the phrase in your question could modify "anger," telling
what anger.
But that idea doesn't work for "fears" and "frustrations." Those are not directed
at someone. We're
afraid of, and
frustrated by. So we must reject that analysis.
So let's go back to the verb itself. Let's say you're venting water out of a hose. Can you vent it
at someone? If you're venting bad feelings, can you vent them
at someone?
Does it work for all three direct objects - fears, frustrations, anger? All affirmative.
If I vent my hose at my friend, does "at my friend" modify the object "my hose" or the action verb "to vent"?
This is probably the hardest question to answer. With some experience, you'll learn that these things modify the verb, telling
how I vent the hose, not
where the hose is pointed.
I'd say that's the situation you have here. "At the bankers and politicians" answers the question of how the action is performed.