A failure to secure the striker to a new contract saw the youngster ferried away by Chelsea.
Vilajoana told AS: "I didn't understand it. I would have done everything possible to keep him. It was obvious that he had a lot of things to improve, but he had a goal-scoring ability.
"He was the type of player who would have developed and improved very quickly here. For a centre forward who shows his face in the youth team and we don't make an effort to keep him...?"
On Guiu's teammate Marc Cucurella, Vilajoana added: "At the time we had to decide, Marc was competing with (Juan) Miranda at left back. The coach was given two options to move up to the first team and he chose Miranda.
"So we had to find a way out for Cucurella. With (Alex) Grimaldo there was no such option because he preferred to go abroad directly."