@sstimson
I thought you considered that idea as dumb :>. Guess you didn't after all.
First, you've missed my point. It doesn't matter he didn't know it's value and the kidnappers knew he didn't know. What matters is he wasn't properly handling it, it wasn't under a protective glass and yet it was whole and unbroken. Plus in the video he was casually holding it and it didn't broke. If it was so delicate that it could almost crumble because of it's own weight, it would when Agasa held it. So not taking it while they've had a chance and instead kidnap a kid that was there to get it later in a ransom-hostage exchange would be really, really dumb and overly complicate things. Especially if Agasa would have to deliver it himself anyway and risk damaging it anyway.
Second, if I were a thief I would grab an item worth thousands of dollars even if it meant destroying it slightly (plus, how hard can it be to wrap a vase in a cloth?). Paintings (even old, priceless ones) being cut out of their frames are common, the profit is still humongous and it makes transporting and getting away with it easier. But that's beyond the point since clearly it wasn't easy to break (as seen in the video), at least not easier then your plain, average jug. Also, you writing that "knowing it is priceless, would not take the chance of it breaking if they took it now" and "They either forgot the equipment they needed, or did not bring enough." makes no sense. If they knew how valuable it was, how old it was and how easily damaged it could be they would surely be prepared. And we've seen their resolve when they kidnapped Ayumi so it wasn't a crime in a spur of a moment, assuming they were thieves (thus I'm starting to think that kidnapping was their goal all along, seeing a beautiful house - thanks to given address or finding it themselves - and seeing he has some valuable items he could be a target but then again there would be a problem of noticing Haibara in his glasses).
Third, you're probably right here partially. I would think they would prefer cash not a vase that would be hard to sell and easy to draw attention of the police.
Because of the above I'm starting to think that they're kidnappers mentioned earlier, not art thieves. Whoever went with that option deserves a cookie (especially the person who thought about it first, sorry don't recall a name).
Edit:
@RedPoppy
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