I had that problem with mine too. I widened the holes that the hinges went in to and that helped. I did this only for the elevators. I used Great Planes white nylon hinges on the flaps and those fit much better than the ones that came with the ARF. All you have to do is widen the hinge pocket about 1/8" and it will fit just fine.

http://www.greatplanes.com/accys/gpmq3950.html

I built an ARC Vector (also a kit Vector) and on every ARF/ARC I have seen the hinge slot is much longer than needed and very narrow, as you observe. Must be cut with a knife-like tool at the factory. I use thin CA at each end of the slot to hold the trailing edge in place for the next step, then use a power Slot Machine or a hand tool like the Robart hinge slot gouges to open the slot to the proper thickness for the hinge tabs. Should be a snug fit but not spread the trailing edge like a snake eating a mouse. I rough up the hinge tabs  with sandpaper and epoxy into the TE, then when model is finished, slide the flap or elevator into place, with more (very thin coat) epoxy. I use vaseline on the hinge pin area to prevent locking the hinge during gluing.

An update from my work this morning.

The pins are splined on one end so that they lock into one hinge half but it's no big deal to push the pins out. The problem eith the individual hinges is the halves bind on each other. Each half swings freely on the pin but the assembly is stiff. A No. 11 blade, good light and a lot of patience gets the hinges moving freely on the pins.

As I am working on two simultaneously, there are 14 hinges on each. I'm happy with the two I've reworked—only 26 more to go.

Either that or I use the Paul Walker solution of one long piece of wire in place of the individual hinge pins. For info, the pins are 1.3mm (0.051")

Mike and Matt have good advice, and it will certainly help your problem. ..... with that said, there is also another solution: 

Robart 1/8" Hinge Points. I have used these on all my planes for decades. They have many advantages over the traditional flat hinges:

1. They are VERY easy to install. Just drill holes! (no slanted slots, etc.)

2. They are ALWAYS straight and smooth working. (please don't use these unless you also buy their great Drill Jig, it makes perfectly straight holes! :-)

3. They are bullet proof and go in with epoxy. I have a 28 LB. 35% plane that is over 10 yrs. old and now has almost 2,000 flights still with the
original Robart hinges.

4. They make several sizes for our 1/2A planes to our 35% planes.

5. They are closer to real scale. From J3 cubs to B737s. :-)

6. They make a hinge pocket for those who want removable control surfaces.

There are "how to" links on this forum, and on their website.

Here is their link:    http://www.robart.com/store/control-hinges?page=1

Once you use these, you will never use any other type of hinge. ;-)

PS: I know you guys know this, but just a reminder for others who may be reading this. Please don't let your friends use CA hinges in our CL planes! EVER!!!! ..... They are OK for small sport RC because the servo can overcome their stiffness. They are a very poor choice for CL. ..... Just when we have min. control overhead on a gusty/windy day is when we really need to have NO drag on our controls! ANY stiffness can be deadly for our plane.