In general, for a given transportation medium, modes of transport that are more densely populated are more efficient than those that are more sparsely populated. For example, it is not a complete comparison to say your Prius gets 50 mpg and the city bus only gets 5 mpg, therefore your Prius is an order of magnitude more efficient than the bus. This is because that bus also moved twenty people at once. So the per-person fuel efficiency of the bus is actually an order of magnitude higher than the per-person fuel efficiency of your Prius.
However, this does not necessarily hold up when comparing modalities from different transportation media (for example, flying versus taking the train). This is because of the differing challenges facing a given medium de facto, such as the fuel spent keeping an airplane aloft rather than propelling it forward, whereas the train is held aloft by static rails.
In deciding on a vacation, how can I determine the more sustainable option between, for example, a train tour of the continent and a commercial cruise to Mexico? The ship likely has an order of magnitude more passengers than the train, but the expense of pushing the hull through water is significant.
Update for 2018
Recent research suggests that vacationing is not only bad for the environment in general, the energy usage from our vacationing is getting worse:
Here, we quantify tourism-related global carbon flows between 160 countries, and their carbon footprints under origin and destination accounting perspectives. We find that, between 2009 and 2013, tourism’s global carbon footprint has increased from 3.9 to 4.5 GtCO2e, four times more than previously estimated, accounting for about 8% of global greenhouse gas emissions.
Thus, if at all possible, find the most ecological means of vacationing.