Having beaten Barcelona in their last outing, Valencia began positively and took the lead after 41 minutes through a header by Jeremy Mathieu.
Alcacer doubled Valencia's advantage on the stroke of half-time, and added his second in the 67th minute, soon after Sofiane Feghouli had made it 3-0. Eduardo Vargas marked his full debut for Valencia by scoring the fifth goal with 11 minutes remaining.
The outcome could have been very different had Leo Baptistao made more of a cross he received inside the area with just three minutes played.
From then on Valencia head coach Juan Antonio Pizzi saw his team make his opposite number Gabriel Calderon suffer.
Vargas and Alcacer missed the target as Valencia found their tempo, and then Chile international Vargas was denied by a great save by Betis goalkeeper Antonio Adan.
But just a minute later Mathieu broke the deadlock, heading in the corner that was conceded by Adan.
And soon after the first goal came the second, Alcacer sticking out a leg to guide a shot by Vargas into the net.
Early in the second half Antonio Amaya went close to pulling one back for Betis, but saw his header saved by Diego Alves.
It was not long before Valencia reaffirmed their command.
Former Arsenal centre-back Philippe Senderos headed over from a corner, and just a minute later the ball was in the net for a third time, Alcacer and Feghouli combining on the counter-attack, with the latter finishing off the move.
Alcacer then got his second goal of the game, heading a cross from Antonio Barragan past the helpless Adan, who had a game contrasting brutally with his debut against Espanyol, in which he kept a clean sheet.
Adan would concede one further goal, Vargas finishing from close range following another quick break.
Valencia reclaim eighth spot from Espanyol with their win but recent form suggests they will have loftier ambitions for the end of the season.
Meanwhile, Betis' hopes of revitalising their season after last week's win over Espanyol appear to have been misplaced. They remain firmly rooted to the foot of the table.