Place the scraps from the outer layers and ends of the onions, carrots and celery in a large stock pot. Add the chicken thighs on top along with a bay leaf, and season with salt and pepper. Add 8-10 cups of water, bring to a boil; reduce heat and simmer until chicken is fork-tender and cooked, about 20 minutes.
Remove chicken and shred; set aside. Strain the chicken broth using a fine-mesh sieve and discard the vegetable scraps and bay leaf; set aside the broth.
Heat the olive oil in the same pot over medium-high heat. Add the onions, celery and carrots to the pot. Season with salt and pepper. Cook, stirring occasionally until vegetables start to soften, about 10 minutes. Add garlic and oregano, and cook until fragrant, about 1 minute.
Return the shredded chicken and strained chicken broth to the pot. Reduce the heat to low and simmer for 10 more minutes. Then add the vermicelli pasta and cook until the pasta is al dente, about 5 more minutes.
Stir in fresh parsley and lemon juice, if desired, and serve warm.