Perhaps, the best thing for the vampires was the fact that Victor had no idea about the ritual they just performed, and except for the altar, nothing else was telling about it. Victor did do a quick scan around the room for any abnormal thing, but he didn't exactly knew what the room looked like to begin with, it was really not possible to tell what was amiss.
The vampires however didn't need to say or explain much, as it was pretty clear that they were the ones who created the thing that cause the gate to open. The problem came when they had to say the reason for the creation of the thing to begin with.
