Put it as your first "House Rule" that the guest will see on reserving your place. Everyone else in your area is in the same boat. I'm sure someone else here will be able to provide verbiage for you to use for that rule.
Yes, it sucks, but best to just find the right way to handle it since it's been months since the problem was identified with Texas and nothing has changed so far.
Same problem here in Breckenridge since October 1. I can't believe there are not more people asking questions. I called CS and they gave me the same answer about noting it in House Rules and then adding an extra charge after they book. I won't do that as I don't want to get it on the other end with a bad review about extra fees after the fact.
I don't want to gross up my rate because that makes me less competitive and actually show up lower on the page. Plus I will be paying tax on the grossed up rate that I would use to pay tax. Also, many individual owners won't even figure out that they are loosing money on this. The big Property Managers seem to still be billing the 12.275%.
My current solution is to note in house rules that the "Administration Fee" is to pay for local tax. I took my average rate for the winter season, calculated the taxes at 5.9% and mark it as an Administrative Fee. Thanks to VRBO you can't make your own custom label. Yes, I will still pay tax on the "fee" that is tax and I won't always collect enough. However, it is better than losing all 5.9% out of the Gross Rental Rate.