I can understand your situation but abruptly stopping Viibyrd may result in withdrawal symptoms. Viibryd causes sedation and may be this symptom "brain-fogged" is because of sedative
nature of Viibryd. If you are taking it in morning, taking it at night time may help.
You have also mentioned that you are having headache, fever which itself may cause symptoms such as lethargy, poor concentration tiredness.
In my view, there can be two possible options in your case -
First is to consult another GP (not your psychiatrist who is not available) and take some medicines for controlling fever and headaches which should be very helpful in controlling your current symptoms.
Second, instead of abruptly stopping Viibryd, one possible option could be to reduce the dose of Viibryd to half and see the effect. Reducing the dose may help in minimizing the side effect.
Best of luck for your thesis viva and exams. Please feel free to ask any related query.