20th level mythic vampire antipaladin is incredibly tough. The problem is that is the chance of the mythic vampire antipaladin failing its save vs command undead is slim to none. They have good will saves, get CHA to saves and have channel resistance. That should put their bonus to save vs command undead at about +36. That means they probably only fail on a 1. If you have mythic improved channel they do have to roll twice and take the worst result. That would give you about a 10% chance of success.
A 20th level vampire antipaladin would be a bit easier but also not as tough. They would still have about a +21 to their save but that is going to be a lot better than +36.
Good luck on that. A vampire antipaladin is insanely tough making it mythic is even worse. You can keep trying but while you are doing that the vampire can attack you. Since an Antipaladin can channel negative energy, they can take command undead. Do not be surprised if the if he steals any other minions you may have.
As a mythic vampire he can add his energy drain to all his attacks including those with a weapon. At 20th level he will be getting at least 4 attacks. He could drain as many as 8 levels in a single round. Any saves vs that will be taking a -2-penalty due to aura of despair.
He will probably have over 400 HP and a AC of nearly 40. His lowest save will be reflex and should be about +19, his normal will save should be about +22. He has access to greater invisibility so can attack while invisible, or he can use dimensional blade to make all his attacks touch attacks.
This is a CR 27 encounter just by himself. If he has minions it will be even worse.