This quote might help you.
That means that the characters that fail the saving throw don't know anything is amiss. If you roleplay the character as having a hunch that it's not real then you are acting like something is amiss. Which is bad roleplaying since the character doesn't realize anything is amiss.
They did know something was amiss, though, the missing kama. Edited to add - and, if they knew the kama was missing and all still failed their will saves, I would think a potential magic item/treasure would be enough to warrant an additional search of the body, which would again fail the tactile/touch element of the spell.
Additionally, I would think beheading the vampire's corpse and burning it would qualify as "interacting" per the SRD. Since the two mentioned illusion spells (programmed & permanent image) do NOT have tactile/touch elements to them, I would give the PCs huge bonuses to their interaction will saves ("You go to strike the vampire, and your stake passes right through it's body like it is still in gaseous form, though the body in front of you looks solid enough.")
Heck, just being close enough to open the coffin would be close enough to interact, as the opener is within melee range, unless you use a 10 foot long crowbar as a lever while everybody else is hiding around a corner.
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