NFP is usually presented as a valid option when one has grave reasons. Wanting to pursue an expensive vacation is often used as an example of a less-than-stellar reason for avoiding children.
Now, I agree with the OP: a vacation can be a wonderful thing for a family, just as it can just be one, big exercise in stress management and quarrel-avoidance.
But the key here is to learn to discern when one is simply being selfish and using “family time” as an excuse to spend lots of money.
Moreover, the Church has given us guidance on what sorts of “grave reasons” are acceptable, one of which is economic.
… but if one doesn’t have the economic ability to have a new child because one insists on setting aside $1,200 per month to save up for a massive European holiday, I would call into question whether or not this is grave.
Moreover, as lots of folks can attest, having a vacation doesn’t need to be expensive! A person doesn’t have to spends thousands of dollars at DisneyWorld, for example, while refusing to take advantage of simpler forms of leisure in their own backyard.
… come to think of it, I think one could find great deals even on a European vacation. My parents are currently in Poland for something like a month. My mother found a little condominium/apartment to rent for something like a thousand dollars for four week. They’re certainly not spending more a few thousand dollars in total, and they’ve saved up out of their pensions and social security income.
So again, I think the key here is maintaining a balance between wanting to vacation and wanting to waste a ton of money to vacation. Insisting on wasting money does not equal grave economic reasons to avoid more kids.