My main craft area is crochet. I recently discovered several "vintage" doll heads at a thrift store and was intrugued enough to buy them. Here's the thing, sure I can crochet a bonnet or hat to cover-make the head, but what if I want to add a head in felt or other material. Is there anyway to seamlessly glue it in place?

I don't understand your question, if you already have the heads. Can you add a picture to show what you have or what you are looking for? Or am I just being dense? Does anyone else have any ideas?

OK, now I understand, I just read your post in the crochet forum. You could try making a cloth doll, including the head, and glue the face to the front of it. I doubt that it would be seamless, though. I think I might be tempted to use the faces as wall decor instead of dolls. What are the faces composed of, ceramic, vinyl?

Anyone else with any ideas?

I was afraid of this when I bought them--the use it as wall decoration.
They are made of plastic.
I did find one way to use them
I hid the fact it was just a face and not an enire head by glueing a crochet bonnet around it. That's okay for babies, but I was hopeing to do more styles of dolls than that.

I love the picture! It's very cute and a great idea for using the faces.

Are all the faces as toddler-like as the one shown? If so there are some really good books on the subject of crocheted toys, which I'm sure you're familiar with. I think it might be interesting to put a doll's face on an animal shaped toy.

If that idea doesn't appeal, I don't see why you couldn't do the same thing you have done in the picture only use a different style hat and a more toddler or young child body for the doll.
There is a wonderful book called Cloth Dolls by Brenda Brightmore that has patterns for dolls of all different ages.
You can read my review of the book here: Review of Cloth Dolls by Brenda Brightmore

The article also has a link at the bottom to buy the book if you wish. I highly recommend the book because it is full of information on cloth doll construction and has great patterns as well.

I hope this helps. Please keep us updated here at the doll making forum as to what you do with the other faces.