Each of the words on this list are, indeed, used in A Separate Peace by John Knowles. My space here is limited, but here goes:
Capacious is a rather formal, or perhaps antiquated word which simply means spacious or containing plenty of room. Knowles uses this word in the first chapter when Gene walks back onto the campus he left years earlier. He describes the houses he remembered when he attended Devon School, grand houses on the best street in town:
...capacious Greek Revival temples lined the street, as impressive and just as forbidding as ever.
Droll means funny in kind of a dryly amusing way. Knowles uses this word to describe Finny's hair when he comes out of the water and his hair is plastered rather crazily onto his head:
...his wet hair plastered in droll bangs on his forehead.
Prodigious means impressively grand in some way, perhaps by size or length or strength. In this novel, Knowles uses it to describe how much effort it takes to jump from the infamous tree into the water without getting hurt:
Standing on this limb, you could by a prodigious effort jump far enough out into the river for safety.
Rhetoric has to do with how something is said or asked more than about the substance of the question. A rhetorical question, for example, is a question which does not require or expect an answer; instead it is used as a stylistic device to make a point. in this novel, the word is used about Finny the dreamer, of course. Finny is
...asking rhetorical questions and echoing other people's words.
Anarchy is another interesting word in this novel. We know the story is about a bunch of boys at a boarding school, the perfect setting for this particular word. Anarchy is a condition in which there is no leadership and the result, of course, is chaos. And who is the inciter of chaos in this novel? Phineas.
Finny’s life was ruled by inspiration and anarchy, and so he prized a set of rules. His own, not those imposed on him by other people.
Solace is a different kind of word, changing tone from rather mocking or disobedient to something more emotional and real. Solace simply means comfort, something offered or given especially in a time of sadness or grief. Once again Finny is the one who needs some comforting after what Gene has done. In this sentence, however, it is Gene who needs some solace. When Gene begins to feel that Finny might be trying to keep him from excelling in school, the thought takes Gene's breath away. He is in "misery," "devastation," and "despair," and he searches for a positive thought to rely on.
Not rely on absolutely, that was obliterated as a possibility, just rely on a little, some solace, something surviving in the ruins.
Vulnerable is a more common word which means able to be hurt, wounded, or defeated. Certainly both Gene and Finny feel vulnerable at times throughout this novel, but it is Finny who lives full-out who is most vulnerable to hurt by the people he cares most about. This is an apt description of Phineas:
He's charismatic, pure of heart, naturally skilled, and most importantly, vulnerable in an almost palpable way.
Books which stand the test of time and are therefore considered to be classics utilize an effective vocabulary, and that is certainly the case with John Knowles in this novel.