The theme of death often juxtaposes the theme of mortality and immortality. The poetic speaker faces her mortality by accepting the pull of death "into the silent land." She seeks to imbue herself with a kind of immortality by requesting that her beloved remember her after she enters "into the silent land," after she enters into death.
Here, the idea of the frailty of human intention enters in as she recognizes--not with rancor or bitterness, only with compassionate resignation--that life will override human intention and produce short intervals of forgetfulness, "for a while."
She attempts to forestall the added grief he will feel because of forgetting--thus rendering her continually mortal--by telling him that another will continue to remember her without interruption. This other that will remember her is the remnant of her thoughts that are left behind in her poems and that, by always remembering her to her readers, will imbue her with the constant immortality she seeks.
This theme represents a standard convention used by poets, including Edmund Spenser and William Shakespeare, who attest that their words, their poems, give them immortality as well as give immortality to the subjects of their poems.
Yet if you should forget me for a while 9
And afterwards remember, do not grieve: 10
For if the darkness and corruption leave 11
A vestige of the thoughts that once I had, 12
Better by far you should forget and smile 13
Than that you should remember and be sad. 14
The original twist that Rossetti gives to this standard poetic convention is that beyond her words (the "vestige of the thoughts [she] once had") giving immortality because they exist eternally, as Spenser and Shakespeare both attest, Rossetti imparts to "the vestige" of her thoughts--to her words--the power of remembrance. This theme of mortality and immortality, then, is integral to the paradoxical resolution that frees her beloved from the task of always remembering--thus providing a form of immortality--by declaring that her words remember, thus give her continuing immortality.