No pulse, no breath, no heat; these are a good start to blending in as a corpse. Extra points for if you're in torpor or its day time. A non torpid vamp at night pretending to be dead would just be more or less laying with their eyes closed trying not to move -- could be susceptible to being startled or shifting in a non-corpse like manner.

I think everything would be good until it got to the autopsy table. Organs are atrophied and weird, no fluids or half digested food or rigor mortis or anything like that.

Without any special bonuses or abilities I'd say they could blend in with other dead bodies or even as a murder victim to someone passing by or doing light investigation but as soon as a doctor starts investigating the jig will be up soon.