Snapshots themselves should have no bearing on your DC's function. Not unless you revert. I am a Senior Consultant with 46 full time outsourced clients and over 100+ block support clients. I have SAME model i use for backups across board using vRanger and it takes a snapshot "WITHOUT" quiescing the VM. I have NEVER had a problem with any client or their AD due to this. I know AD inside and out which is why i post this because based on the problem the two cases should not have issues. Now mind you that i ALWAYS run a ntbackup of system state scheduled to run prior to vRanger so if i have to restore DC from vRanger i can and then restore system state.
I hear people say its easier to promote a new one. Well it is and its not. What if you have other components on there like the CA or license server. Also if you want to keep the same name of the DC, god i hope no one replies "use ntdsutil to cleanup metadata" as that should not be a task to do on a day to day and should be reserver only for when you can't properly demote DC or restore as no matter how good it is your still ripping into the database and removing objects in an unnatural way which is why there is 10 other tasks you need to do along with that.
I am curious what made you conclude that snapshots were the cause of your issue? What troubleshooting was done? Why could they not login:
was netlogon paused
was the DC advertising
was there issues with secure channel
was there DNS issues
etc..
I am wondering what led to assumption it was Snapshot and not something else... (I am not saying it was not snapshots dont get me wrong, but i work on facts and am curious to find out what happened with your system and what led you to conclude that?)