If you know these people then why not ask them as opposed to anonymous people on LRC? Or do you not really know them and are basing a lot of this off of assumptions? It just seems odd that if you know person X and person Y, then perhaps you might know a little more about their lifestyles and how they are able to live them the way they do. Perhaps they are fortunate enough to tack vacations on to research related travel (conferences, field work, etc...).
If the money is coming from their parents (a likely source), then choosing how it is spent is their prerogative. Perhaps the parents genuinely want them to be able to travel and see the world while they are young. I don't see how spending money someone gave you is wrong unless of course there were stipulations placed on receipt of that money and its use. There is also the off chance that they have won some prestigious scholarships out there. I have a friend who won a very prestigious scholarship during the 2nd year of his PhD to the tune of $50,000/year for three consecutive years. Granted he used a good chunk for research, travel and networking to land himself in the position he is in now, but he also did have plenty left over.
Also, as someone who just finished his PhD and did not vacation internationally, I would be jealous if I know other grad students who were doing what you described for purely entertainment reasons. I'm not saying it's wrong, as I would do it myself if given the opportunity, just that it sucks sometimes to watch others live it up so to speak when you are not able to. But, we all have things to be grateful for in our lives and some are not as measurable as others. For example, I live in Canada in a cold region and am just accepting now that the next 5-6 months will be full of blocked driveways from snow plows, plugging in my vehicle every night, snow shoveling core workouts that I didn't plan, treacherous commutes, and no vacation plans... but, I do enjoy my job and fiance.