Does the Catholic Church see in Constantine's vision a divine mandate or simply a myth?
In 2012 Pope Benedict XVI wrote:
A month from now we will celebrate the seventeen-hundredth anniversary of the appearance to Constantine of the Chi-Rho, radiant in the symbolic night of his unbelief and accompanied by the words: “In this sign you will conquer!”
so I don't think it's just a myth to him, Pope Benedict seems to consider the vision as Constantine's guiding light out of doubting Christ (i.e. night of unbelief).
Visions from the 4th century aren't vetted with the same effort that miracles today are. I doubt the average Catholic has to put any more faith in that statement than in God saying to Francis "Rebuild my Church" and both could easily be misinterpreted.
The Pope Emeritus further writes (to Eastern Churches):
... remember the promise made to Constantine: “In this sign you will conquer!” Churches of the Middle East, fear not, for the Lord is truly with you, to the close of the age! Fear not, because the universal Church walks at your side and is humanly and spiritually close to you!
So it could be that "conquer" means the same same thing Jesus says to the disciples, you'll stick it out to the end - which is a defacto conquering because if you make disciples of all nations; if the gates of hell do not prevail against you; and if Jesus is with us to the end of the age; you have conquered.
This is not strictly answering your question (posed as how the Catholic Church considers the vision), but I'll just throw it in just in case this can add value. This is considering from Constantine's own perspective, historically vetted and interpreted.
Rather than divine mandate, Constantine would have regarded the vision he received and the subsequent victory as the justification and vindication of the power of the Christian God. Because at that point he was not yet a Christian, so why would he took orders from a god he didn't believe in?
But he wouldn't have taken it as a myth either. Being practical as he was like other military generals in his period, who prayed to their own gods for military victory, this vision + victory would have been taken as "proof" that this Christian god was a true god, perhaps the only god. His opponent of that celebrated Battle of Milvian Bridge (Oct 28, 312) was Maxentius, a rival emperor claimant who reportedly consulted oracles in the pagan temples in Rome. Because Constantine won the battle, wouldn't it be surprising he took the Christian God as more powerful? Then there is the coinage evidence with the Chi Ro affixed on top of the labarum (see paper below).
As for the REAL story of how Constantine took it and his subsequent conversion and actions, when evaluating the contemporary accounts of this event (Lactantius's On the Death of the Persecutors, Eusebius's Ecclesiastical History and the subsequent revision 10 years later, and Eusebius's Life of Constantine we'll have to take it with a grain of salt, since there can be elements of embroidery and elaboration.
To research more, I found this full treatment of Constantine conversion from a university student senior history paper: http://www.wou.edu/history/files/2015/08/Tyler-Laughlin.pdf. It's also treated at length in lecture 16 of this Teaching Company course: https://www.thegreatcourses.com/courses/fall-of-the-pagans-and-the-origins-of-medieval-christianity.html