You can make a lighter non compliant PFD which you fill with other inflatable items, eg Platypus bottles (I carry a 1 and 2 litre bottle, pillows (I carry the Exped Ultralight), wine bladders (who doesn’t have a few of them lying around?) and etc.
Here is the link to Mountain Laurel Designs ‘Thing’ or ‘Mopacka’: https://web.archive.org/web/20100403230340/http://www.mountainlaureldesigns.com/shop/product_info.php?products_id=141  which weighed 4 oz (114 grams) not including its flotation ie the Platypus bottles or inflatable pillows (eg Exped’s Ultralight pillow: http://www.theultralighthiker.com/exped-ultralight-pillow/) but you carry them anyway!
Some folk have made their own. See these two discussions (I have ‘borrowed’ their photos for reference purposes – I hope they don’t mind. Thanks guys): http://packrafting.org/forums/viewtopic.php?t=880 Wine bladder PFD: http://bushwalk.com/forum/viewtopic.php?f=37&t=23122
MLD ‘Thing’
MLD ‘Thingâ