According to the DVA site, you are NOT suppose to provide you DVA account number to third parties. I have always done it myself.
From the DVA FAQs:
Can I use my Disney Vacation Account to make a payment on a reservation booked through my
travel agent?
Yes, you can use your Disney Vacation Account to make a payment on a reservation booked directly with Disney through participating travel agents.
Ask your travel agency whether your booking will be eligible to be paid for with Disney Vacation Account before making your reservation. However, only you may make a payment with your Disney Vacation Account (please refer to the following question for payment instructions).