PFDs are often pretty heavy. Alpacka have this one http://www.alpackaraft.com/product/astral-v-eight-pfd/ at 554 grams which is (I imagine) about as light as they get. I discovered that inflatable PFDs you buy from boating supplies shops have an airline PFD inside them. When I stripped one down it weighed 282 grams as shown and should be adequate for the job.
You can (though not legally) go lighter. You can utilise an inflatable vest such as the Aerovest or Xerovest (at about 60 grams) as I did on the Seaforth. They are a bit awkward to let down again and are really not intended for the purpose.
As I have mentioned before Erin McKittrick (in her ‘Long Trek Home’: http://www.groundtruthtrekking.org/Journeys/WildCoast.html) used a converted Thermarest which she had cut a hole in for her head, and fastened it with a belt. Given that you will need a sleeping mat anyway, this option means that your PFD maybe weighs next to nothing. You should explore this option further if you want to save more weight. The prospect of cutting down one of eg Klymit’s pads for the purpose but keeping it usable for sleeping also appeals.
Mountain Laurel Designs used to make a thing he called ‘The Thing’ which allowed you to utilise your Platypus bottle as part of a PFD system.
I suspect Alpacka’s ‘Fiord Explorer’ & etc seats could be modified slightly to make a light (non-compliant) PFD. They weigh 224 grams without the straps and buckles which would be needed, so it might not be worth the trouble compared with the first example.
Another option would be to buy some of the waterproof nylon which Klymit etc use in their products which sticks to itelf with a hot iron – and make your own.