Reasons for these two past perfect tenses

I had once vowed that I would never call her aunt again: I thought it no sin to forget and break that vow now. My fingers had fastened on her hand which lay outside the sheet: had she pressed mine kindly, I should at that moment have experienced true pleasure. But unimpressionable natures are not so soon softened, nor are natural antipathies so readily eradicated. Mrs. Reed took her hand away, and, turning her face rather from me, she remarked that the night was warm. (Jane Eyre)

Are the two past perfect because of the past tenses below, or because of some other reasons?

(Past tense)--------------------(the past of the Past tense)
I thought it no sin-------------I had once vowed
Mrs. Reed took her hand away----My fingers had fastened

Answer

I think you've got it. The Reference time (narrative time) here is the sequence of moments marked with the past tense. The past perfects mark their events as occurring before the corresponding past moments:

                      NARRATIVE TIME
I had once vowed           before   I thought it no sin ... now
My fingers had fastened      before   [her hand] lay outside the sheet^✲
[a counterfactual and
a general observation, and then]        Mrs. Reed took her hand away, and ... remarked

^✲Actually, her hand may be presumed to have lain outside the sheet before Jane took it ... but in the sentence what is relevant is that it lies there in Reference time.