I think you can get it to work as 2checkout has a system similar to WorldPay where you can add a password to the callback URL. Probably best off confirming that on the vb forum or even submitting a support request to vb support. I have always gotten a response within 24 hours.
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vB3 does not yet support 2CheckOut and I'm not sure what their plans are (if they have any) to support it in the future, however you should be able to make your own module for it, depending on how it works. Try just looking at the WorldPay module and see how it is done - it may be possible to customize it to work with 2CheckOut.