I could use a little more info on that myself That message actually comes from a quote from someone else who looked into it, not me.
Here's what I know. "Vampire Embrace MCA Patch" was written to be compatible with the version of MCA 5.2 that includes Undead ... which is to say, if you are using MCA 5.2 that is compatible with "The Undead 3.0", then the patch is not compatible. The reason it is not compatible is that it has some references to the Undead creatures in MCA 5.2 that got removed in the version that was made compatible with the separate "Undead 3.0". You could probably find out which references those are by loading "Vampire Embrace MCA Patch" along with MCA 5.2 (Undead 3.0 Compatible), and seeing what errors go into your Warnings.txt. These errors will probably tell you about references to non-existant creature IDs. Now that you know those IDs, you can search for them in "Vampire Embrace MCA Patch", and remove any scripts or whatnot that refer to them. Or, you could try contacting Deylendor and see if he has any plans for another version of his patch that is compatible with the new version of MCA 5.2.
Personally, I just never found it to be much of a problem, I just don't load the patch and avoid enthralling MCA's spawned NPCs.