Can I trigger a survey for a specific variation of an existing A/B Test campaign in VWO?

Yes. Using custom trigger, you can display a survey for a specific variation of an existing campaign in VWO. Use the following code snippet in the custom trigger:

function() {
window.VWO = window.VWO || [];
VWO.push(['onVariationShown', CAMPAIGN_ID, VARIATION_ID, function () {
executeTrigger();
}])
}

Replace CAMPAIGN_ID and VARIATION_ID with your campaign id, goal id and variation id. Goal ids are available in report against each goal name. So goal G1 has a goal id of 1, goal G2 has an id of 2, and so on. Variations ids too, are available in the report against the variation names. So variation v1 has a variation id of v2 (Control has the variation id of 1), and so on.