The 21-time Dutch champions have had a tough start to the season and find themselves 10th in the Eredivisie after seven league games without a win.
It was all square at the break but Stijn Schaars' departure due to injury early in the second half heralded a swing in momentum as first-placed Vitesse scored five goals after the interval.
PSV – who had not conceded six goals at home since a match against Twente 21 months ago – are now 13 points behind the league leaders but Cocu is confident he can turn it around.
"I've seen a team that wants to compete with ambition and experience," he said. "That's the benchmark (and) as long as this exists, I will believe in this team.
"The players gave their all and put in an excellent performance. But we could not convert our chances into goals and conceded a soft opening goal.
"We got back into the game and scored a fully deserved equaliser, but things went from bad worse when Schaars had come off.
"We lost grip in midfield and could not find our rhythm any more. I hoped we could snatch a late draw when (Karim) Rekik had pulled a goal back, but it was all over when we were left to rue another defensive slip.
"Looking at the table, we're where we deserve to be."
