Psalms 42:3 in the Vulgate has:
Emitte lucem tuam et veritatem tuam. Ipsa me deduxerunt...
Why is it ipsa and not ipsae?
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When a pronoun refers to multiple things that don't have a natural gender, it's not uncommon to use the neuter plural. Since the light and the truth aren't naturally feminine, and the words with their grammatical genders appear in a separate sentence entirely, it can be more natural to refer to them as illa "these things" than to refer back to the genders of the words.
See Allen and Greenough 287.a for some examples with adjectives, such as Cicero (Fin. 3.39):
Stultitia et temeritās et iniūstitia…sunt fugienda.
Folly [f], rashness [f], and injustice [f] are to be shunned [n].
I can't find a specific citation for this use of pronouns in A&G, but in my experience it's significantly more common with pronouns than adjectives (since they're typically farther away than their referents).
Draconis is right that the neuter plural is possible, but that's not the whole story with this passage. This translation (and remember that all of the Vulgate is a translation) comes from the Septuagint version of Psalms. If you check that, you'll see that ipsa is translating the Greek αὐτά. Moreover, "light" in Greek is neuter (φῶς), and so using the neuter plural is appropriate there.
Jerome later re-translates the Psalms from the Hebrew instead of from the Greek, and he ends up with what you expect:
mitte lucem tuam et veritatem tuam
ipsae ducent me....
Jerome's initial translations follow what he considers is the sanctity of the text itself, as he mentions in De Optimo Genere Interpretandi:
Ego enim non solum fateor, sed libera voce profiteor me in interpretatione Graecorum absque scripturis sanctis, ubi et verborum ordo mysterium est, non verbum e verbo, sed sensum exprimere de sensu.
For I myself not only admit but freely proclaim that in translating from the Greek (except in the case of the holy scriptures where even the order of the words is a mystery) I render sense for sense and not word for word. (trans. Fremantle)
You can furthermore read the Prologue to Hebrew Pslams here:
Herein he provides the immediate context for the new translation.
Therefore, because recently, when disputing with a Hebrew, you produced certain testimonies about the Lord Savior from the Psalms, and he, wishing to outmaneuver you, asserted throughout nearly every one of the words that it is not found thus in Hebrew, so that you were opposed to the (version of the) Seventy interpreters, you most zealously demanded that, after Aquila, Symmachus and Theodotion, I translated a new edition in Latin speech. For you said yourself to be greatly confused by the variety of interpreters, so that you are inclined by love to be content with either my translation or my judgment.