I'm not surprised that biking in VFF's can cause plantar fasciitis--much of the energy from your legs is spent deforming your foot instead of moving the pedals. In cycling shoes, stiffer means more comfortable as well as more efficient.

I would ask you to consider a high tech maximalist approach to cycling footwear. Modern road cycling shoes are stiff, lightweight, and well-ventilated. The retail prices are admittedly expensive, but you can watch for specials and closeouts at sites such as Bike Nashbar and Performance Bicycle. As others have mentioned, Keen and Shimano have open-toed options that accept mountain bike cleats.

The main drawback to cycling shoes is that they're not made for walking, especially road shoes. "Rideability" and "walkability" tend to be inversely related.

I may be taking a total stab in the dark, but what about a pair of chuck taylor converses? I cycle in those and haven't had any issues. The low-cut ones are pretty minimal as far as shoes go, have a stiff & flat bottom, and look pretty shnazzy to boot.

You wear VFFs because your feet were already designed to walk/run properly. However they weren't designed to efficiently push a little peddle over and over again with great amounts of force with a goal of rotating a wheel and delivering as much force as possible through your leg with every pedal stroke. Cycling in VFFs is like running in your old shoes. You are using the wrong tool for the job.

A minimal cycling shoe is great. They are very light and expensive. But incredibly stiff. Stiff = efficient. If you are going to keep using flat pedals, look at some shoes made just for that. Some lightweight downhill mountain bike shoes designed for flat pedals would work great. You can read reviews to gauge the degree of stiffness but if you are seated most of the time you won't notice much of a difference as long as they are decent shoes.

Assuming you don't have clip in pedals. I don't either. I've used Chrome bike messenger shoes for a couple years now. They have nylon reinforced soles to transfer power to the pedal and they look pretty cool, too.

http://www.chromebagsstore.com/shoes.html

I remember a sandal company had sandals that were cleat-able. I'll see if I can find what they were. It was about 10 years ago in Portland, OR.

Edit: It looks like Teva had a flip-flop that was cleat-able. Here's an example with egg-beater cleats. You could get something like this, and maybe poke a few more holes in the sole to strap it to your foot a little more securely than just the flip-flop straps.

I'm also seeing a lot of stiff-looking leather sandals, such as here and here. Those looks stiff enough to bike in, but still zero-drop or close to it.

The problem with going too minimal, of course, is that the sole won't be thick enough for the cleat (I'm assuming you've got a cleat system, be it spd or egg-beaters) making walking on the other end difficult. There are some other less minimal options that still give your toes some air.

I've been thinking about this a lot lately, too. I have been walking for my commute, but I'm racking up too many miles and my plantar fasciitis has come back. I'd like to start biking again, but my toes no longer fit in my old bike shoes, and my huaraches don't have enough thickness to comfortably sit on top of the cleat. So thanks for this question, because it's reminding me I need to get new sandals to put my cleats on.

While commuting I wear chacos. Otherwise for longer rides, clips are it. You need to efficiently transfer motion as a function for being a part of the machinery of movement. Therefore, clips become more necessary the longer you ride.

I bike 4 times a week, about an hour each trip. I'm more of a enjoy-the-hawks-and-flowers kind of rider than a Tour de Whatever cyclist outfitted in an aerodynamic Micro-Max jersey. Recently I switched to a bike which allows me to sit with my back straight most of the time. So if you're that kind of bike rider, you might find the following helpful.

I wear SoftStar shoes, RunAmocs, with the 5mm trail shoes. They feel great. If you go with SoftStars, be sure to get the 5mm soles. I tried wearing my 2mm RunAmocs but with those I could feel the pedals too much.

I much prefer RunAmocs over the Merrell cross-training shoes which I had been using the previous years.

I use clips, and I wouldn't bike without them. I think going minimalist on a bike doesn't make sense... Our feet evolved to walk and run, not to bike. It is a very unnatural foot position, so why would you use minimalist shoes?

My suggestion is to get a good pair of mountain bike shoes (more comfortable than road, and typically have big cleats so you can walk in them) and a pair of egg beaters.