Is anyone currently using vtiger CRM 5 and successfully integrating their CRM with CampaignMonitor's API?
Is it possible to integrate CampaignMonitor's API with Vtiger CRM ?
A web search would suggest that a Campaign Monitor add-on or integration for vtiger probably hasn't been done before.
There is no reason why such an add-on or integration couldn't be built though.
This could be your opportunity to build it!