Flintridge Prep jumped out to a 3-0 lead in the top of the first, but a pair of errors led to a quick Vasquez retort in the bottom of the inning that saw the Mustangs tied the game at 3.
"We're sitting pretty and [then] defensively, we're horrendous," Cook said.
After a two-run third for the Mustangs brought the score to 5-3, the Rebels never led again en route to the loss, which kept Prep from a playoff win for a stretch that dates back to 1999 and put Cook at 0-8 in the postseason.
"Maybe it's something I'm doing," Cook said.
But for the 2011 Rebels, they ended their season after a second-place finish in the Prep League at 14-10 largely due to their inability to shore up the exact area of the game that their coach warned them about.
"Unfortunately, our defense didn't help us today," Cook said.
After Vasquez (18-3), which took second in the Desert Mountain League, went ahead, 5-3, Prep kept coming and tied the game at 5 in the sixth before the Mustangs surged back ahead with a three-run sixth.
Kyle McDonald walked and later scored Prep's first run of the opening stanza before a Steve Fleming single brought in two more for a 3-0 lead.
Prep had a solid opportunity to get back into the game in the top of the fifth, but the miscues spilled from the field to the base paths. Despite notching a pair of hits and Royce Park getting hit by a pitch, the Rebels came away with nothing thanks to two outs on the bases.
"If we don't run ourselves out of the inning, we get something going," Cook said.
The Rebels collected a pair of runs thanks to four straight singles in the sixth, but had the bases loaded with one out and their Nos. 3 and 4 hitters coming up in Park and Karlsen Termini, but could get no more than the two runs.
"We couldn't get a hit," Cook said.
In large part, Cook put the comeback effort on the shoulders of his freshmen, as six of his team's 11 hits came from ninth-graders.
"It would've been ugly if our freshmen didn't step up," Cook said.
Freshman Brenden McKiernan, who had a pair of hits and a pair of RBI, and freshman Mike Weinstein were responsible for the run-scoring hits in the sixth. Freshman Daniel Enzminger also had a pair of hits.
McKiernan drove in a run in the seventh, as well, drawing the Rebels to 8-6 with a two-out infield single to score Fleming, who went two for three with two RBIs. But a groundout to shortstop ended the Rebels' year a pitch later.
Park got the start for the Rebels and lasted 4 1/3 innings, allowing five runs.
On the bright side for Cook and his staff is that Prep will graduate just one senior this season.
"From a coaching standpoint, you look at everyone coming back and, if we can keep everybody healthy," he said, "we should be fine."