The sealing ritual he used was the one that Heinz taught him when he previously decided to seal his Golden Pocketwatch.
The Sealing Ritual itself wasn't that strong, according to Heinz, but perhaps, because of Vale's unusual connection with the Spirit World, the seal that he made had a different level of intensity.
Vale had noticed how effective it was, especially when he tried resealing the Pocketwatch that was slowly recovering its power.
However, it seems that he had underestimated the Divine Sword. The seal barely lasted for a week, and Vale had to redo the sealing ritual.
This time, Vale prayed really hard, hoping that the sealing ritual would get stronger that way.
Whom~
Vale suddenly heard a familiar sound confirming that the ritual was completed. The Formation Circle he made had also disappeared confirming that the seal was successful.