Cook the pasta al dente following the package instructions. Drain, do not rinse or use any oil.
While water is coming to a boil, in a large saucepan saute the chopped onions and pancetta until tender over medium heat, for about 5 mins mins.
Drain the clams, reserving 1/3 cup of the juice. Add the drained clams, reserved juice, and the red pepper flakes to the saucepan, and simmer for 2 mins. Stir in the vodka sauce and parsley and heat gently.