As a growing medium, vermiculite's main purpose is water retention. Many potting mixes contain vermiculite. Including FFOF (Fox Farms Ocean Forest potting soil) which I use. With mild summers and decent rain fall, you could easily get by with watering once a week. I grew autoflower in a 3 gallon pot, in Southern California, and watered about once a week. With your climate and a 5 gallon pot, two weeks may very well be possible. Don't get too hung up on that lid idea, the suggestion of just using a layer of mulch sounds better.
Most people who grow in 5 gallon pots are growing plants as big as the pot will allow, usually 5 foot and bigger plants. Of course those will need to be watered more frequently. But you're growing a plant that will average 18 to 24 inches. Big difference in the amount of water needed.
Take the guess work out of it. Fill a 5 gallon pot full of your soil mix, plant a tomato, bell pepper or whatever, completely soak your soil, and see how long it takes between waterings.
The autoflower grow I did in the 3 gallon pot last summer is in the bottom link in my signature. That grow is towards the end, so you may want to start at the last page and work your way backwards.
Best of luck.