Vacation mode is a low-standby setting, not a true off switch. Instead of holding your water at the usual 120 degrees and reheating every time the tank cools, VAC mode keeps it at a much lower temperature, commonly around 50 to 60 degrees. The unit fires only enough to keep the water from going fully cold, which cuts standby energy use while leaving you a warm-ish tank you can bring back up to temperature quickly when you get home.
How you set it depends on whether the heater is gas or electric. A gas water heater usually has a dial on the thermostat labeled with a VAC or vacation setting between Low and Off. Turn the dial to it and the pilot stays lit while the burner barely runs. Electric units, especially newer ones, often have a digital control panel with a vacation button or mode. Older electric heaters without that feature can be set to their lowest temperature for the same effect.
For a long trip, a week or more, turning the unit fully off generally beats vacation mode. There's little point keeping any standby heat going when nobody's home for many days. The small fuel or electricity savings of VAC mode over fully off no longer justify running the appliance. On a gas heater that means setting it to Off or Pilot, and on an electric heater it means switching off the dedicated breaker at the panel.
If you do turn the heater off for an extended absence, consider shutting off the water supply to the house as well. A water heater holds 40 to 50 gallons, and a tank or supply-line failure while you're gone for two weeks can flood a home. Closing the main shutoff, or at least the cold-water valve feeding the heater, removes that risk entirely, and it's cheap insurance for a longer trip.
Arizona adds its own wrinkle, especially with a garage water heater. In a Phoenix-area garage that hits 110-plus degrees in summer, the surrounding air does a lot of the heater's work for it, and the tank loses far less standby heat than it would in a cold climate. That makes vacation mode less necessary in summer. On a short summer trip you may find that simply leaving the heater at its normal setting costs almost nothing, since it barely has to fire to hold temperature.
Whatever you choose, the most important step is on the way back: return the thermostat to about 120 degrees Fahrenheit. That setting is hot enough to discourage bacterial growth like Legionella in the tank yet not so hot it scalds or wastes energy. Give the tank an hour or so to recover before you expect a full hot shower, and if you shut the water off, reopen the supply slowly and let the tank refill all the way before relighting or powering the unit.