Hi.
I had this problem yesterday and google got me to this thread 
which is already couple years old.
I use Esxi 5.5U2 in 2015 and vCenter showed 99% for hours ( I also have to say that it jumped to 99 suddenly from maybe 50 percent ) .
I rebooted vCenter according to this thread.
Out of curiosity, in between I logged into the host directly.
But when I logged directly into the host where the machine was running it showed 85% and the number was increasing untill it really finished.
BUT it somehow was randomly changing to 99% from time to time and then going back to real state ( 85......86.....88..... etc )
So I was thinking that maybe vCenter misbehaved on this.
Anyway. The result was that I didn't have to reboot the host and everything completed. It just took long time.
I don't really know if the reboot of vCenter was necesarry for me. Next time if this situation happens ( I hope not )
I will log directly to host and see what it shows and if vCenter at least stops misbehave once the deleting of snaphost is really over according to the information from Esxi host.
Tomas.