Q. When the Rite of Acceptance is celebrated at Mass, what happens to the collect?
A. I think it’s just something that the framers of the RCIA didn’t think through. The Rite of Acceptance was surely designed to be celebrated apart from the mass. For example, the priest or deacon may wear a cope (48), but not a chasuble or dalmatic. However, at the end of the rite, the option for continuing with the liturgy of the eucharist is plainly appended (68).
In the Roman Missal, the various masses for Christian initiation begin with the Rite of Election, not with a Rite of Acceptance into the Order of Catechumens. Again, just an indication that the missal never envisioned a collect would be needed because it thinks the Rite of Acceptance is a word service, not part of a eucharist.
Pastorally, I think there are many good reasons for celebrating the Rite of Acceptance at Sunday Mass with a congregation. In that event, either the concluding prayer (57) replaces the collect for the mass of the day, or the collect replaces the concluding prayer.