Edgar Markov’s eminence ability is a triggered ability. Edgar must be on the battlefield or in the command zone when you cast another Vampire spell and also as the triggered ability resolves. If it’s on the battlefield or in the command zone when you cast another Vampire spell but leaves that zone before the ability resolves, the ability won’t do anything as it resolves.

Notably, if Edgar Markov is on the battlefield and its eminence ability triggers, but it’s put into the command zone before that ability resolves, that ability won’t do anything as it resolves. This is because an object that changes zones is considered a new object.