It depends on the type of vacation. I often travel alone and to destinations that require a lot of moving about from transit to transit. As such I try to get away with just an internal frame pack (mine is a Gregory Diva) and a tote bag. I've never been denied carry on with the Gregory pack on an international flight and on domestic flights the worst they've done is check it at the gate with that little tag and bring it out when I get off right away. I like the pack because it makes it possible to walk around the city easily. I tried to do that once in NYC with a large rolling duffle and I'm pretty sure I wanted to murder things after 30 minutes. In my case, I prefer not to spend money on taxis though.