If I recall correctly, the prices in the dealers book, lowest to highest, were:
employee
vendor
invoice
retail (sticker)
My employer has an agreement with Ford, Dodge and GM that allows me to get the vendor price. No haggle.
When I bought my 2008 GMC there was a special promotion where everyone gets employee pricing. I just checked KBB, which claims that in 4 years I have lost 2k in value.
Try asking what employee and vendor prices are. You probably will not get an answer. That being the case the best you can probably do is work up from invoice. Never work down from sticker.
If they won't tell you what invoice is then just look it up online.