Is there any Church ruling, discipline, etc. that requires candidates and catechumens to be brought into the Church during the Easter Vigil, or, instead can a parish decide to bring people into the Church throughout the year whenever the pastor feels each person is ready?
Except in grave circumstances, confirmation is to be done by a Bishop. However, as the Bishop can not be in many places at once, he authorizes Priests to do confirmations @ Easter Vigil for those who the Pastor believes prepared via RCIA and on other times upon request.
It is unlikely that the Bishop would approve the arrangement you suggest without certain compelling circumstances. The reason is that there is great symbolism in combining Confirmation with the Easter Vigil. We are an Easter people who rise again because of Christ’s Resurrection. There is great practical reasons as it allows the Candidates/Catechumens to walk the RCIA journey consistent with the Church Liturgical Year.
The Period of Inquiry – This is when people can ask questions to get an idea about Catholicism. They learn about scriptures and Jesus Christ. This period takes as long as it needs to. When the person is ready to go to the next phase, sometimes they need to write a letter saying why they feel they want to convert. This period usually lasts up until right before Advent when most Parishes have a Rite of Welcome to bring the candidates into the parish in universal preparation for the coming of our Lord @ Christmas.
Period of the Catechumenate – For this phase to begin, people need to accept Jesus Christ is their savior. They begin going to Mass on Sunday and further their commitment to the faith. They reflect more deeply, about how Catholicism will affect your life. This phase also lasts as long as it needs to for individuals. This period usually ends prior to the beginning of Lent.
Period of Purification or Illumination – This period corresponds with the time of Lent. It is the six-week preparation time for Easter. It becomes the praying time for those who are to become Catholic known as the Elect. The period is begun by the Rite of election, usually celebrated at the Cathedral Church with the Diocesan Bishop. This rite accepts them accepted as candidates for the Sacraments by the Bishop, representing the fact that this decision is not theirs alone. Normally this rite takes place on the first Sunday of Lent. Special prayers are offered to support the Elect.
Celebrating the Sacraments of Initiation – The Sacraments of Initiation is celebrated at the Easter Vigil. The Easter Vigil is an extended night watch of prayer, singing, and hearing the Word of God. By the waters of baptism, a person passes into the new life of grace and becomes a member of the Body of Christ. Those who are all ready baptized, have the Sacrament of the Reconciliation prior, are given Holy Communion, and are confirmed. Anointing with Holy oil called chrism seals the initiation by the power of the Holy Spirit and participation at the Table of the Lord in the Eucharist marks full membership in the church.
The Period of Mystagogy – This period lasts from Easter Sunday until the completion of the Easter season. That is fifty days later on Pentecost Sunday and completes the initiation process. Those who have just shared in the sacraments of initiation are now called Neophytes and during this period of Easter joy they reflect on what they have just gone through and look to the future as to how they can now share in the mission of Christ who came to bring salvation and life to the whole world. This period reminds the whole church that life in Christ constantly calls us to grow and to look for new ways to live the life of grace, personally and together.
Why would you want to do it any other way?