Hi Ross,
Vegas does not have a corner-pin effect. Since you're from Imagineer Systems, I assume you know that corner-pin data from Mocha can be loaded into Boris FX BCC Corner Pin for Vegas Pro and used in Vegas. If you wanted to write something that loaded corner-pin data into Vegas natively you could use Track Motion keyframing. To my knowledge Vegas uses bi-linear when rendering in "Good" mode and "bi-cubic" when rendering in "Best" mode.
~jr
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