Is this the same nun in these sutta references?
They seem to be two different persons.
Info on Vajira Theri:

Vajirā Therī. The Samyutta Nikāya (S.i.134f) relates that one day, when she was taking her siesta in Adhavana at Sāvatthi, Mārā
questioned her as to the origin of “being&quot; (satta), its creator, its
origin, its destiny. Vajirā answers that there is no such thing as
“being,&quot; apart from certain conditioned factors, like a chariot, which
exists only because of its parts. Māra retires discomfited.
Vajirā's verses are often quoted (E.g Kvu. 240, 626; Mil. p.28;
Vsm.ii.593) both in the Canon and in later works, but they are not
included in the Therīgāthā, nor do we know anything else about her.

Info on Sela Theri:

Selā Therī. She was born in Alavi as daughter of the king: therefore
she was also called Alavikā. When she was still unmarried the Buddha
visited Alavī with Alavaka, whom he had converted, carrying his
begging bowl and robe. On that occasion Sela went with her father to
hear the Buddha preach. She became a lay disciple, but later, agitated
in mind, she joined the Order and became an arahant. After that she
lived in Sāvatthi. One day, as she was enjoying her siesta in the
Andhavana under a tree, Māra, in the guise of a stranger, approached
her and tried to tempt her. But she refuted his statements regarding
the attractions of lay life, and Māra had to retire discomfited
(S.i.134; Thig.vss.57-9).
In the time of Padumuttara Buddha Selā was born in the family of a
clansman of Hamsavatī and was given in marriage. After her husband's
death she devoted herself to the quest of good, and went from ārāma to
ārāma and vihāra to vihāra, teaching the Dhamma to followers of the
religion. One day she came to the Bodhi tree of the Buddha and sat
down there thinking, &quot;If a Buddha be peerless among men, may this tree
show the miracle of Enlightenment.&quot; Immediately the tree blazed forth,
the branches appeared golden, and the sky was all shining. Inspired by
the sight, she fell down and worshipped the tree, and sat there for
seven days. On the seventh day she performed a great feast of offering
and worship to the Buddha (ThigA.61f). Her Apadāna verses, quoted in
the Therīgāthā Commentary, are, in the Apadāna itself, attributed to a
Therī called Pañcadīpikā, and are twice repeated in these verses
(Ap.ii.519, repeated at 527f), however, she is mentioned as having
attained arahantship at the age of seven, and there is no reference to
her life as daughter of the king of Alavī.