The instituted acolyte does not take vows. But he requests the ministry in writing to his bishop. Parts of the ceremony of institution are like vows. For example, the bishop’s prayers include:
“5. … Let us ask him to fill them with his blessing and strengthen them for faithful service in his Church.”
“6. … Grant that they may be faithful in the service of your altar and in giving to others the bread of life; may they grow always in faith and love, and so build up your Church.”
“7. … Make your life worthy of your service at the table of the Lord and of his Church.”
(The Rites Volume Two, Liturgical Press, 1991, ISBN: 0-8146-6037-1, pages 108-109).
The ceremony of institution is one of the sacramentals. The Catechism of the Catholic Church discusses these in n. 1672.
“1672 Certain blessings have a lasting importance because they consecrate persons to God, or reserve objects and places for liturgical use. Among those blessings which are intended for persons - not to be confused with sacramental ordination - are the blessing of the abbot or abbess of a monastery, the consecration of virgins and widows, the rite of religious profession and the blessing of certain ministries of the Church (readers, acolytes, catechists, etc.)”
(From vatican.va/archive/catechism/p2s2c4a1.htm ).
So it is not a job like doing the flowers or mowing the lawn. In some sense the man has been consecrated to God through the ceremony. So he cannot quit without incurring guilt. He is not instituted for three years (for example) but is making a lifelong commitment.
For Extraordinary Ministers the appointment can be for one Mass or a period of time. This is explained in the 2004 Instruction Redemptionis Sacramentum:
"[155.] In addition to the ordinary ministers there is the formally instituted acolyte, who by virtue of his institution is an extraordinary minister of Holy Communion even outside the celebration of Mass. If, moreover, reasons of real necessity prompt it, another lay member of Christ’s faithful may also be delegated by the diocesan Bishop, in accordance with the norm of law, [footnote 256: Cf. Code of Canon Law, can. 230 § 3.] for one occasion or for a specified time, and an appropriate formula of blessing may be used for the occasion. This act of appointment, however, does not necessarily take a liturgical form, nor, if it does take a liturgical form, should it resemble sacred Ordination in any way. Finally, in special cases of an unforeseen nature, permission can be given for a single occasion by the Priest who presides at the celebration of the Eucharist.[footnote 257:
Cf. S. Congregation for the Discipline of the Sacraments, Instruction, Immensae caritatis, prooemium: AAS 65 (1973) p. 264;
Pope Paul VI, Apostolic Letter (Motu Proprio), Ministeria quaedam, 15 August 1972: AAS 64 (1972) p. 532;
Missale Romanum, Appendix III: Ritus ad deputandum ministrum sacrae Communionis ad actum distribuendae, p. 1253;
Congregation for the Clergy et al., Instruction, Ecclesiae de mysterio, Practical Provisions, art. 8 § 1: AAS 89 (1997) p. 871.]"
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from romanrite.com/girm.html :
“107. The liturgical duties that are not proper to the priest or the deacon and are listed above (cf. nos. 100-106) may also be entrusted by a liturgical blessing or a temporary deputation to suitable lay persons chosen by the pastor or rector of the church.”
This does not include the instituted acolyte, who is described in n. 98. It is not up to the priest to choose him. The priest does select the altar servers, who are described in n. 100:
“100. In the absence of an instituted acolyte, lay ministers may be deputed to serve at the altar and assist the priest and the deacon; they may carry the cross, the candles, the thurible, the bread, the wine, and the water, and they may also be deputed to distribute Holy Communion as extraordinary ministers.”