I really, really hate hearing about TAs like this. As a TA it saddens me on so many levels - for the poor family that's left without a vacation, for the bad reputation it gives other TAs, the whole thing is just not fair.
Let this serve as a reminder to everyone to be sure you know who you're dealing with. Don't let this discourage you from using a TA - there are unscrupulous people in every profession. Just make sure that you research/interview a prospective TA. A GOOD TA will make your planning easier and take the stress OFF you. A GOOD TA is an invaluable resource that will be in your corner fighting for you should anything go wrong, will keep an eye out for potential problems and will watch out for your best interests. Get recommendations from your family, friends, coworkers, folks here etc on good TAs to use. Several very good and very reputable TAs are mentioned frequently on these boards.
It's no guarantee of ethical behaviour, but when talking with a TA ask if they're certified (CTA, CTC or CTM). This means that they've put the time and effort into professional development and education and have completed several exams in order to become certified. In Canada, for instance, out of more then 33,000 TAs there are only 2,100 that are certified.
Also ask what organizations the agency is a member of (eg ASTA, ACTA, CLIA). To be honest it doesn't say much about the integrity of the agency, as these organizations will give membership to just about anyone who pays the fee. But it at least says that the agency was willing to pay the fee to join.
Your vacation will cost thousands of dollars, and result in a lifetime of memories. It's a shame that unethical TAs can make those memories painful ones. Please be sure that you're using a TA that will truly make your vacation the experience of a lifetime.