I would appreciate if anyone can help. I will depart IAD (Washington Dulles) for CDG Paris, be arriving at terminal 2F terminal at 12:00 noon. I have to make a connection flight to Saigon VN at terminal 2C at 13:00 with Airfrance. What is a quick way to go from 2F to 2C within one hour window (besides wearing a pair of running shoes). Also, someone mentioned to me that I have to go thru customs..to get to 2C, why is that? I thought I should be in the transit area for all these international segments since I am flying on Airfrance all together. Please help. Many thanks.

You don't have to go through customs to get to 2C; follow the signs for transit passengers. There is a bus that will transfer you to 2C. However 1 hour is rather tight, given AF's propensity to be late, so it would be a good idea to inform your FA that you have this connecting flight.

I called Airfrance today, the rep recommended that I should go thru the passport check and go out of terminal 2F then walk to 2C, but I have to go thru passport and security check again as entering 2C. She said, it would take me around 45 minutes for this. I dont know if this is the quickest way, but your advice of informing FA is a very good idea. Thank you for your advice.

You might want to do a search on the AF forum about transfers--there's a lot of info there. Consensus is that 1 hour is tight because of all the things that can go wrong (late flight, not getting a gate but parking on the tarmac and being bussed to the terminal, etc.) It even makes a difference if you're in F or C vs Y (being in the back of the plane can add fifteen minutes to your connecting time just waiting for 250 people to get off ahead of you). And your luggage might not make the connection, even if you do (so be sure to have everything you need for a few days at your destination in your carry on).
The airside bus goes clockwise around the airport--2B, 2F, 2C, 2A (I think it skips 2E because it's still mostly closed and it skips 2D because that's a "domestic" [Schengen zone] terminal). So 2F to 2C is only one stop and probably faster than walking (figure about ten minutes to walk plus time to clear passport control in 2F and passport/security in 2C).
Finally, talking to a flight attendant might help, especially if you don't get a gate. Sometimes AF will have a small shuttle for connecting passengers waiting on the tarmac if you don't get a gate or an agent at the gate who will take you to a shuttle. Good luck.