It doesn’t look like VerneMQ exports metrics in any application-specific way, so I’m not sure how much of an advantage you’d get out of a native Telegraf plugin. One thing a dedicated plugin would provide is the ability to identify which applications you’re running, for use with canned Chronograf dashboards, for example.
If you’re interested in a plugin for a particular application, you can always open an issue on GitHub. The Telegraf team might decide to prioritize it, or someone else from the community might pick it up—in fact, the most common way that a new plugin gets created is through community contributions. A large portion of our plugins have been built by members of the community who were using an application and wanted to easily get metrics from that application into InfluxDB and decided to write something themselves.