Just because you're on vacation doesn't mean that the laws of physics and the realities of driving with an unrestrained child change.
Before deciding on bringing a car seat, first check your airlines' policies on this subject. It's usually on the website. If not, email (don't call) them and get the skinny on it.
Also, check your car seat. I can say universally, if you're in Canada or the U.S., it's airline approved (very few on the market aren't). There will be a sticker somewhere on the seat saying that it's airplane approved. By contrast, if your seat requires a shoulder belt to install it, then it probably isn't, as is often the case in Europe. Airplanes only have lap belts. The manual will solve this mystery if you're unsure.
Then, research your destination. Any way that you can avoid taxis and personal cars while there? Is there a shuttle to your hotel? In some places, local buses aren't secure so visit elsewhere on this site and find out. If you can avoid small vehicles, even if your child is not restrained, you still have the statistics and physics on your side.
The lap belt on an airplane isn't going to restrain a 2 year old very well so it's much safer to have the car seat on board. But if you can avoid road travel in general, than taking the "risk" in the airplane might be worthwhile. Commercial air travel is very safe so there's little chance of anything going wrong (the logic behind lap babies).
Never use a car seat from a car rental company. Many parents claim they're "fine" but there's no way to really know. There's also a hygiene issue with these seats.
Never check a car seat as luggage. Bringing it on board and using it is the smartest plan. If you can't do that, gate-checking is preferable to luggage checking. Remember, car seats are only built to withstand ONE accident.
I simply put my car seat on a small, metal luggage cart and pulled it behind me as I went through the airport. They sell specific gizmos for the same purpose but they're basically just overpriced luggage cart.
I'm a former Flight Attendant and I fly a lot with my three kids. Lots of flying tips on the net but few from actual airline people. Mine is totally non-commercial (I'm just sooooo altruistic ;)
http://flyingwithchildren.blogspot.com