Whenever Cordial Vampire or another creature dies, put a +1/+1 counter on each Vampire you control.
If Cordial Vampire dies at the same time as one or more other creatures, its ability triggers for each of them.
If a Vampire you control is dealt lethal damage at the same time as another creature, they die at the same time. That Vampire can't receive a counter from Cordial Vampire's ability in time to save it.