Hi Rupietoot
You have two chances to get the Viazul bus from Jibacoa..Slim and None! You are about 40 minutes +/- from Matanzas in the wrong direction from Jibacoa, so just bite the bullet and arrange a cab or catch a ride with someone else that might be touring to Havana on the day you are going.
Have a Great Time and good luck!
F&T