I'd never heard of Croque-anything until I took a French class in college and was suddenly having to order them from my classmates, along with a café au lait and croissant. I don't know what made me think of them recently, but I suddenly decided I wanted a vegan version. This meant digging through recipes online to see if I could find something workable. A few vegans have beat me to the punch and created their own vegan Croques. Of course, I had to share.
By the way, I saw a couple of non-vegan, vegetarian recipes (heavy on the gruyere) that used mushrooms and/or spinach instead of vegan ham, so you might consider substitutions like that if you don't want the faux ham that some of the recipes use.
There aren't very many recipes, so people, get on this!
~Parsley Soup's Vegan Croque Monsieur
~The Vegan Croque Monsieur with Dijon Dill Aioli at Veganize It...Don't Criticize It
~D.I. Wine and Dine's Vegan Croque Monsieur aka Savory French Toast
~The Vegan Croque Monsieur (with a side of Dijon Potatoes!) over at Yummy-Licious
~Shins' Vegan Lovin' has a Vegan Croque Madame Panini
~Animal Emancipation showcases a Croque Madame...no real recipe but you should be able to figure it out with all the ingredients listed.