Typically on the vintage machines there is no way to lower the feed dogs. I would not even consider removing the feed dogs to free motion quilt.
Most people cover the sewing area with a plate or plastic, leaving just a hole for the needle; that prevents the feed dogs from coming into contact with the quilt sandwich.
Have you tried free motion machine quilting while the feed dogs are up? This would be while using a darning foot (which "hops" from stitch to stitch so feed dogs are fully engaged only while stitch is being made, and have less influence between stitches). Some people actually prefer free-motion quilting with the feed dogs up as it can help keep the stitches even.
Experiment with a practice sandwich and see which approach is going to work best for you.