From my experience, a lot of the realty sites iframe the "virtual tours".
The agent only has to submit a link to where the tour is hosted.
For example, the local mls site does this...
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<iframe style="border:hidden; height:600px; width:800px" src="http://link.tothetour.com/whatever/elsecan/gohere/etc"></iframe>
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And in the case of realtor.com... they use that "red house" icon and just link to the "tour" to show in a resizeable pop-up window.
It will vary from mls to mls... but for it to be compatible across all of the systems... the agent is only submitting a link to the tour... then each site displays it however they like.