The rear opening of the C8 is 37-38mm. If the field stop diameter of your 2" eyepiece is greater than this then there will be some vignetting although it will unlikely be noticed because of the position of the rear opening in the light stream. Eyepieces with field stop diameters greater than 38mm include 31mm Nagler (42mm FSD), 41mm Panoptic & 55mm Plossl (both 46mm FSD).
A 35mm Panoptic has always been the recommended eyepiece for a C8 since its FSD is 38.7mm.
See
https://www.celestron.com/blogs/know...the-ota-itself
Note I always saw the C8 rear opening reported as 38mm yet the Celestron post above says 37mm.
Whilst 2" eyepieces are one way to get low power on your C8, you do need a 2" visual back and 2" diagonal. Note too the effective focal length of your telescope becomes greater with a 2" rear assembly than with a 1.25" assembly. A better way to get low power on a C8 is to use a 0.63x focal reducer/field flattener screwed to the rear opening followed by a 1.25" visual back and 1.25" diagonal. Then use say a 24mm Panoptic which has a 27mm FSD and you will get the widest field possible and at the design focal length of the telescope (x0.63 because of the FR/FF). So in effect your 27mm FSD becomes 27/0.63=42.8mm without the vignetting.
Note three 1.25" eyepieces can give you the widest true field; Tele Vue 24mm Panoptic, 32mm Plossl & 40mm Plossl. Each has 27mm FSD but gives different powers and exit pupils.