You can invert the mask to get everything except the vessels, then mask:
% Get fundus mask, which is the opposite of the vessel mask
mask = ~vesselMask;
% Mask image by multiplying each channel by the mask.
maskedRgbImage = rgbImage .* cast(mask, 'like', rgbImage); % R2016b or later. Works for gray scale as well as RGB Color images.
% For R2016a or earlier, mask the image using bsxfun() function to multiply the mask by each channel individually. Works for gray scale as well as RGB Color images.
% maskedRgbImage = bsxfun(@times, rgbImage, cast(mask, 'like', rgbImage)); % R2016a or earlier.
