Q: Question. Planning mass for the Easter vigil. Is it possible to replace one of the responsorial songs with a motet that would use the actual words of the psalm but not in a response fashion.
A: Let me give you the whole quotation of paragraph 306 of my book, Let Us Pray.
The GIRM refers to the psalmist as a role distinct from that of the cantor and the reader (61, 99, 102, 129, 196, 352), but in the absence of a psalmist, a cantor or reader may lead the psalm. The documents never mention a choir singing the psalm. This may be an oversight, or it may have to do with the preference that the psalm be led from the ambo (GIRM 309; LM 22), since the choir sings from another place (GIRM 312). However, the ambo should be large enough to accommodate several ministers at the same time (LM 34). In practice, some choirs assist on verses of the psalm, and the scriptural texts of the psalm sometimes presume that several voices are singing. The gradual, which may substitute for the psalm, requires the assistance of a choir (Ordo Cantus Missae 5). The entire assembly would logically assume the role of the psalmist when the responsory is sung in metrical forms (GIRM 61). Normally, though, the GIRM has a psalmist in mind. The psalmist should sing skillfully (GIRM 102), but so should the cantor. The ideal separation of the role shows the distinct ministry of the one who leads the psalm, which is integral to the proclamation of the Scriptures in the Liturgy of the Word.
That said, I think you can justify a choir singing a motet. The missal also allows silence to replace the psalm during the Vigil.
But I wouldn’t overlook the beauty of having the assembly sing the psalms. It keeps them engaged while listening to a lot of words at the Vigil.