Have you tried not taking your meds one day to see how you feel? If you aren't used to it, or have been taking Vyvanse for a while, your ADHD symptoms suddenly coming back might be hard to deal with. So I'd suggest that before you decide to do anything.

And as always, I would advise you talk to your doctor about it if possible. Some people can experience withdrawal from Vyvanse, even if they aren't physically dependent:

Physical dependence (a state of adaptation manifested by a withdrawal syndrome produced by abrupt cessation, rapid dose reduction, or administration of an antagonist) may occur in patients treated with CNS stimulants including VYVANSE. Withdrawal symptoms after abrupt cessation following prolonged high-dosage administration of CNS stimulants include extreme fatigue and depression.

(source: Shire prescribing guidelines [PDF warning])

Other than that, I think you should weigh up the pros and cons - You may get less anxious without medication, or you may get more anxious. You may also have trouble sleeping due to ADHD symptoms - staying up too late trying to cram in revision etc. Your ADHD symptoms without meds may make sitting the exam more difficult too - getting distracted, not keeping track of time, not keeping your thoughts in order, rushing through questions etc.

Something I've just realised myself though - Vyvanse is effective for a maximum of 11-14 hours, so really the best way to have your cake and eat it, would be to take it as early as possible.