I've had the same experience.  I was over 200, dropped my calories to 1200-1300 intake per day, and 30 pounds melted away.  Then I started training for a half-ironman and the pounds started creeping up and eventually plateaued.  With my training I was working out 6 days/wk, sometimes 2 workouts a day and for the last 5 months have been hovering between 180-185.  I am a vegetarian, have been for over 10 years, so like you, my diet is very similar.  I have seen 2 nutritionist, and I am still working with the 2nd, but anyway, this is what they have come up with.

Because my energy ouput went up and my caloric intake was too low, I wasn't buring fat anymore, my body was eating away my muscle.  And my body was retaining more of what I ate b/c the lack of nutrition.  So once I raised my calories, the pounds starting dropping, very slowly.  Also, are your run times getting slower or faster?  I find once I stopped worrying about my weight and shifted my focus to performance, the weight started dropping slowly again.

All in all, be patient, in due time everything will come together.  Put your mind toward your running and the weight will follow.  Good luck!

Don't be surprised when you raise your calories, your weight may go up at the start, but it will come back down in due time.  Also, as to not get discouraged because you can't see the number of your weight go down, start taking body measurements.  Take measurements of your arm, chest, waist, hips and thigh, for example.  That way, you can see how your muscle is toning.

Also, don't dramatically increase your speed or the mileage of your runs so you can lose the pounds faster.  This most certainly will lead to injury.  Week after week, start adding another 5 minutes or pump up the speed the last 1/2 mile, these little bursts will ease you into gradually increasing your runs.