it says nonrefundable....so ask them nicely if you could cancel -- but the reason you offer here is not compelling to the extent "not feeling confident" is not a reason to default on a contract.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.
Based on the information provided, it seems that the 'non-refundable booking fee' would be kept in the event that you not only transfer/postpone/change the date, but if you desire to cancel the services altogether. This would be the logical interpretation of the contract. In other words, all monies paid (minus the booking fee) should be returned to you upon notifying the vendor of cancellation. I think this is your best avenue to pursue considering how close you are to the wedding.
I doubt that you would have strong legal claim to have 100% of the monies returned, even though you feel less confident in their abilities (to a judge, that just usually means greener/cheaper pastures somewhere else). What have they done, other than slowing communication? When was the last time that you actually did communicate with them? Months? Weeks?
Essentially, 'cancelling' the contract without a good reason is breach of contract on your part and possibly may leave you liable for money beyond the deposit. If the contract is as vague as you state, then the vendor will probably have a strong argument that they are entitled to expectation damages, or the money that would have been in their pocket (minus expenses) had you not cancelled. It's likely that this is less than the full contract amount, but more than the deposit. Considering that you want to cancel about two months prior, the vendor could have a strong argument that it is entitled to such damages.
Disputing the charge when it inevitably occurs will probably be unsuccessful. I see wedding vendors win those claims left and right.
Mr. Schenk may be reached at (678) 823-7678 during regular business hours, or anytime by email at firstname.lastname@example.org. The above statements are provided for general information purposes only and are not intended as legal advice or advice of any sort for a specific case or legal matter. If you do not have a signed attorney-client fee agreement with The Schenk Law Firm, then until such written fee agreement is provided and signed by both a prospective client and attorney for a particular case, neither Mr. Schenk nor the Firm will represent you nor will they be your attorney in any matter and you remain responsible for retaining your own attorney and for compliance with any and all deadlines and for any statutes of limitations that may pertain to potential claims. Comments made on a public forum, such as Avvo.com, to not have any confidentiality because others may read them. If you desire a private consultation with Mr. Schenk that is confidential, please go to www.schenkfirm.com or call (678) 823-7678 for a free initial consultation.
If you are still on good footing with this vendor you could try amending your contract with some terms that will make you feel more comfortable. Based on your question I'm not sure what that could be, but the best solution may be to just create a higher level of confidence and at the same time install some terms that you can rely on if you need to go to conciliation court later. Next time don't pay in full up front, you lose all leverage. Good luck.
Providing this general response does not create an attorney client relationship.