Shortly after Hope and William Gagnon arrived in Cancún, Mexico, from Detroit, the couple were approached by a salesperson from a vacation club. About 21 hours later, they had agreed to pay $27,000 to join the club — a significant commitment for Ms. Gagnon, a customer feedback specialist, and Mr. Gagnon, a hospital courier.

They had bought what they believed the Unlimited Vacation Club was selling: free or deeply discounted weeklong stays at luxury resorts with opulent suites, pristine beaches, swim-up bars and private concierges.

But the Gagnons learned after they signed the contract that the club had numerous restrictions. The unavailability of many resorts, hidden fees and a requirement to pay extra for upgrades that would give them access to the swankiest rooms made it nearly impossible to book the kind of vacation that had impressed them during a pitch meeting.

“I was like, my God, we got had,” Ms. Gagnon said.

Vacation clubs are the latest twist on timeshares — an industry often criticized, and even mocked, for using aggressive sales tactics to persuade people to buy a share of a vacation property, giving them the right to use it for a certain number of days a year.

In a vacation club, there is no actual real estate ownership. Instead, people make a down payment to join, followed by monthly payments, to get discounted rates at a portfolio of resorts.