I always look vintage linens over carefully before buying to see what kind of shape they're in...if there are any frayed threads or excessive wear and tear. I also keep in mind what I plan to do with them~~will they be getting washed frequently or are they just for display. Sometimes the condition of the item decides what type of use it will get.
All of my embroidered pillowcases get washed on a heavy duty/long cycle in warm water with regular detergent (I swear by Tide because I have 2 boys who are dirt magnets and usually look like they've wrestled pigs by the end of the day.) The first wash of all of my vintage pillowcases includes a splash of bleach, then I only use bleach when they start to look dingy. I'm sure this is a huge no-no but I've never had any fall apart (knock-on-wood!) And I haven't noticed any excessive fading with the threads either. I also just throw them in the dryer with the rest of the sheets (and I only iron them for photos, lol!)
I will add that because I have a number of pairs of vintage pillowcases, I rotate them so each pair probably only gets used every 3-4 months.
The current set of pillowcases that I'm stitching are pre-printed. With plain pillowcases, I've done: iron-on patterns, traced patterns using a light box, and also traced patterns with an iron on transfer pencil (if you're unfamiliar, you trace your pattern onto tracing paper using an iron on transfer pencil and then you iron the pattern onto your fabric~~this will reverse the design.)
I only use DMC thread when embroidering pillowcases (or anything else that will be washed.) And I've never had any trouble with the color bleeding.
I hope this helps you :) I can understand anyone's hesitation about using vintage/embroidered pillowcases, I used to feel the same way. But I've come to realize that even if something fades or wears out~~the bit of joy I get every day from seeing and using them is far greater then knowing that they're tucked away in the closet.
As a post script......this isn't how I treat all of my vintage linens, just the bedding ;)