frankthedm
First Post
If a player casts Dominate Person spell, that takes a one round casting time. When a Vampire uses it's Dominate Supernatural Ability, it only takes a standard actionActually, in your example, the target of the Dominate Person would be unaffected completely by the Dominate Person.
When a spell has a casting time of "1 round", it doesn't mean it takes a full round action and then it's cast. It means it takes a full round action, and then it's cast at the start of your next turn.

Dominate (Su): A vampire can crush a humanoid opponent's will as a standard action. Anyone the vampire targets must succeed on a Will save or fall instantly under the vampire's influence, as though by a dominate person spell (caster level 12th). The ability has a range of 30 feet. At the GM's discretion, some vampires might be able to affect different creature types with this power.