If you want to take out the supernatural from the vision of Constantine, as an amateur historian I will say that by Constantine's era Christianity did not need saving - it was winning - especially in the east.
Constantine came from the eastern part of the empire and was surrounded by easterners even though his base was in the western empire. So even if he was not making a cynical play, he was already aware that Christianity was on the rise in the eastern empire and maybe he was sympathetic to the religion already and susceptible to signs.
If you are cynical and claim no signs were seen, that works too. Constantine knew his base of power had to be in the healthy eastern part of the Roman empire. The western empire was crumbling. In order to overthrow the current power structure of the empire which had an advantage over his smaller forces he needed to appeal to an untapped base of support that was NOT loyal to the empire in the east - the Christians. The Christians at this time made up a very wealthy and powerful base.
Not much is mentioned on this but by this late stage in the Roman empire, Christian businessmen had acquired a reputation for honesty and good business practices and steadily grew in wealth. Roman persecution against Christians was strongest a generation before Constantine and failed badly. A smart power hungry Roman from the east would have taken notice of such untapped potential.
And many of his troops were already painting the Chi Ro on their shields prior to this event.