We have these two tags with the same description:
One question even has both tags.
It should be synonymed the other way around. The product was originally VMS and then it became OpenVMS which I believe it is still called. Other than that I don't see any real reason not to do this.
Creating synonym is overkilling it: it needs 5 users with a score of 5 on the said tag to be done, or a moderator (the last one is very busy) compared with a single retag that requires 2 minutes and a single user. It's better to just retag them and be done with it, which seems sensible considering that nobody has asked a vms tagged question in years, it's unlikely to be recreated.