I never understood the concept of victim souls.
Well, the concept of "victim soul” starts with the Roman Catholic view of suffering.
That, in turn, begins with the Gospel. Everyone shares in the Cross of Christ (to one extent or another) as part of the process that leads to sharing in the Resurrection of Christ. This is basic to Christianity, as the New Testament explains. See, especially, the words of St. Paul, St. Peter and Christ Himself about sharing in sufferings and persecutions for the sake of the Gospel.
Next, the Roman Catholic understanding of suffering encourages people to make an offering of their suffering to God. St. Paul wrote about offering his own sufferings to Christ by His grace and for the good of souls. "Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in the afflictions of Christ on behalf of His body, which is the Church [ . . . ] "
St. Paul's Letter to the Colossians 1:24. This is not seen as substituting Christ's passion but rather participating in His one and only suffering and sacrifice for our sins. In short, we are to ask the Holy Spirit to divinize ourselves and our days by making them an offering unto Christ, and offering our suffering as a gift to God is an application of that principle. That is why you can hear Catholics saying "offer it up" when faced with an annoyance or with suffering. They are offering their suffering to Christ, to be included in His suffering and resurrection and for the good of souls. In Christ, all suffering can be redemptive.
Now, some people in this life suffer egregiously. It seems that the purpose of their lives is to suffer. For example, the woman you mentioned (Bl. Alexandrina Maria da Costa) is one such person. She broke her spine when she jumped from a window in order to escape a rapist. As a result, she ended up crippled and in pain for the rest of her life. She didn't want that. Indeed, she prayed mightily for a cure or at least some relief. She got neither. So (and this is what begins to make her a saint), she chose to not become bitter or depressed. Instead, she chose to accept this as, somehow, the will of God for her. And, as part of that, she offered her suffering to Christ for the good of souls.
That is a victim soul. A victim soul is a soul who, in this life, suffers egregiously and who offers that suffering to Christ in union with His own suffering and resurrection for the good of souls. Again, this isn't trying to substitute or add to His death and resurrection. This is asking Christ to make one's own suffering to be worth something, to be redemptive, by the power of Christ's suffering and resurrection.
In short, being a victim soul is a kind of martyrdom. It is suffering for the sake of the Name of Jesus Christ, by the power of His strength and suffering and resurrection.