It may depend on the wording for the type of spawn. But I see nothing that would take mythic away from the creature by adding the vampire spawn template. If it were a sorcerer it could still cast spells, so if it were mythic it should still be mythic.
I'd say the - at best - defeating a mythic PC should count as a mythic trial... for the vampire. As in, the vampire should power up. Its spawn shouldn't be mythic unless said vampire has an ability that makes its spawn mythic.