Add flour and garlic to the pot and cook, stirring, for 1 minute. Pour in Marsala and cook, stirring, for 1 minute more. Add broth and bring to a simmer. Reduce the heat to low, cover and cook, stirring often, until thickened, about 5 minutes. Add vinegar and the reserved chicken, along with any juice. Return to a simmer. Season with additional pepper, if desired.