Hi
We have not come up against a bacillus cereus case in guinea pigs on here; it is gram positive so trimethoprim-sulfamethoxazole (TMS; better known as bactrim or septrin or derivates), chloramphenicol or baytril (enrofloxacin) are the antibiotics that can be used in guinea pigs but that is basically it. It is unfortunately resistant to many antibiotics.
Vomiting (which guinea pigs cannot do), nausea and diarrhea are the usual symptoms and in many cases, most humans get over it within 2 days or so of rather violent and painful acute food poisoning but it can get a lot worse than that.
Bacillus cereus is well known for causing food poisoning in humans, especially in rice (not properly stored or cooked), meat and other foods. Have you got a dry, cool but not cold place for your guinea pig staples or a safe source for guinea pig food and hay as the problem could also have originated with a supplier or seller and not necessarily with you?
I feel for you but I actually think your vet has read up on it and knows what they are doing; as hard as the bad news is for you (and for them as well, as no vet likes being helpless).
I truly wish I had better news as well.
You could try US-based Guinea Lynx Forum as a last resort, which specialises in medical cases, to see whether there is further information on bacillus cereus, as they are more likely to have had an enquiry for your rather unusual health issue from other people in your unfortunate situation.
However, it is not a friendly place and I strongly doubt that they know of a miracle cure, either.
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Our own UK-based forum specialises in general care, species understanding etc. and in home care and moral owner support during illness and loss, but due to the much stricter regulations in our country we cannot replace any veterinary care.
I am very sorry that I don't have better news and that you were not dealing with such a nasty bug in the first place.
Hopefully someone who has seen this will come along 
