Yunior is referring to the time when his father first took him to the house of his Puerto Rican mistress. This wasn't long after Papi bought the van. Papa regularly takes Yunior out in the van as he claims that it will cure the boy of his traveling sickness.
Unfortunately, it doesn't seem to work as Yunior always ends up being sick at the end of each journey. Still, Yunior enjoys these little trips as they're pretty much the only time he and his dad get to hang out together. It's notable that when they're alone Papi always treats Yunior much better than he does at other times.
But then that's probably because Papi doesn't want Yunior to tell his mom about the Puerto Rican lady. By taking Yunior for regular spins in his van Papi is cynically giving himself a pretext for seeing his mistress. Taking Yunior out in the van to cure his travel sickness is certainly much less likely to arouse suspicion than if Papi just headed out in the van on his own. So despite Papi's treating Yunior nicely on their regular excursions Yunior is being used by his old man to hide his adulterous affair with the Puerto Rican woman.