Vaessen went down after a collision with Ajax forward Brian Brobbey, causing panicked reactions from his team mates as they frantically called for medical help.
Screens were put up around the goalkeeper to shield him from the public eye and players from both sides watched with tears in their eyes as a defibrillator was rushed on to the field.
Vaessen lay motionless on the field for minutes before he was carried away on a stretcher.
After the match had been suspended, with Ajax leading 3-2, RKC director Frank van Mosselveld told broadcaster NOS that Vaessen had regained consciousness. RKC later confirmed that Vaessen had been taken to hospital.
"Following our update last night, we can report that our goalkeeper Etienne Vaessen had a good night and has continued his line of recovery," RKC said in a statement on Sunday.
"For now, it is important for Etienne to recover peacefully in the coming hours and days. We would therefore ask that he and his family be given the time and rest to do so."
