This is a question best left for your accountant. I know you would need to ask for a form showing their TE status, (I know this from my hotel days), but since I've not ever booked a TE group in my VR, and since I've never worked in Accounting I do not know what our accting dept did after I gave them the form from our groups.
ACCEPT THE RESERVATION.
Even if they're tax exempt, they have accepted the rate and costs in order to book, so you're in the clear.
If they have tax exempt status they want to assert, get their info, then later refund any taxes VRBO sends you as part of their remittance (the 3% fee will get refunded from VRBO when you do the refund, so that part's a wash).
The group may not be exempt from tax. This is not tax advice of course, but I work at a non-profit and book hotels for our conferences. We are tax exempt in our state. When we go outside the state, we may or may not be exempt, and we may have to file for a tax-exempt certificate for the state we visit, which we may or may not get.
Our exemption is for sales tax, and may not extend to lodging tax or other similar non-sales taxes. In the state where my VR is located, we have a lodging tax. No one is exempt from paying it.
My advice as a fellow owner would be to accept the reservation, which presumably includes the taxes. Your guests are always able to show a receipt to the state and request a refund from the state. I would not lower the rate in anticipation of not taking taxes out of their payment.