I think they are a big deal to them, enough of a deal to be complaining to neddy frequently, and that's the problem. You don't have to be Mr. Supportive, telling them that they just got unlucky, they never could have known etc. would be lying. On the other hand don't be confrontational or accusatory. If you politely change the subject every time they bring it up, they should get the picture that you don't want to hear about it eventually. If they don't, there's nothing wrong with bringing it up one day, saying that you're sorry that they got a bad van but you don't want to hear about it anymore.
That being said, if they complain about it in ways that imply you should have warned them about it, go nuts. Have a field day with the lecture
