Let me see if I understand the argument people are making. Are you saying that the situation should have played out as follows?
Player: “I think this body is a fake”
{He fails his Will save}
Player: “No, on second thought it looks pretty real.”
If that’s the argument your making, it sounds pretty logical. But I think my counter argument has some validity as well. Let my try to break down my counter argument as simply as I can.
We believed the body a fake with no out of character knowledge and before Will saves were even called for. The key question is “what was the reason for us to believe the body was a fake” The answer is the missing kama.
Now, the failed Will save may help to allay our characters suspicions. But if the reason we initially called the body a fake (i.e. the missing kama) continues to persist. Why would we not continue to think the body was fake?
The only argument I can think of is that our characters initially saw the missing kama as proof of a fake, but after the failed save we were forced to rationalize why it wasn’t there. Which brings us back to the original point of discussion, can a failed Will save force our characters to make such rationalizations.
The answer is yes it can - that is the entire point of such things. Your characters must in fact do so. This is not a compulsion effect this is an character knowledge thing. When acting with knowledge that the character does not have you end up with the metagaming claim. The characters have no knowledge (in fact they have knowledge to the contrary thanks to the failed save against illusion) that the body in the coffin is an illusion of the vampire.
The failed will save against the illusion had your characters believe that the body looked and felt (evidently the way your DM rules illusions - which I disagree with by the way, but he has consistently done so in your game so you as players know that is how they should be played) just like the intended illusion (in this case the vampire).
So the characters believe that the body looks and feels just like the vampire.
The only question is what happened to the kama?
Searching for the kama is a logical path and methods used to do so can be logically (and in-character) rationalized - but attempting to prove the body is an illusion can not.
Checking to see if it was polymorphed is not the same as checking to see if it was an illusion - although a tad shaky on justification it is still within the realms of possibilities to the PCs.
I don't think I would have had any problem with the PCs asking for a detect magic to check for trickery (in essence looking for the kama) if they had not specifically mentioned to check for an illusion - that is the one thing they are supposed to be convinced it is not at this point in time.
Although looking at polymorph - it probably wouldn't have been possible to do this. A dead body is not a willing living target - and if the vampire and subsequently killed the creature such marks would have been potentially seen as well as the duration of the spell itself (1 min/level) makes it an unlikely possibility.
Polymorph any object on the other hand - but it is an 8th level spell. I don't know what level of character the group is looking at since that information was not supplied.
A spellcraft check could have been used to determine on-going effects also - see Rules Compendium pg 138 for the latest ways to use the skill (a really, really useful skill by the way). It can also be used to determine objects shaped or created by magic when studied.
How long has this group of players gamed together?
How long as this been your DM?
Depending on the real life relationships it might be worth looking for a different DM/group becasue of how this DM runs certain aspects of the game (like illusions) - but that is something different to the subject at hand, the other 2 questions help to lay the groundwork for how long the players have had to get acustomed to how the DM runs his game.
How did the PCs know the kama was magical?
How much experience have the PCs had in dealing with vampires?
These are 2 other questions that are relatable to how much player knowledge versus PC knowledge is being used in the game. All of this can have an effect on how your DM feels the game is going and potentially lead to why he got so adjutated.