1. Install `ptvsd` using the same Python interpreter Qgis uses.
E.g. on Windows:
```
C:\OSGeo4W64\apps\Python37\python.exe -m pip install ptvsd==4.3.2 
```

2. Add a debug configuration to `launch.json`: 
```
{
    "name": "Python: Remote Attach",
    "type": "python",
    "request": "attach",
    "port": 5678,
    "host": "localhost",
    "pathMappings": [
        {
            "localRoot": "${workspaceFolder}",
            "remoteRoot": "${workspaceFolder}"
        }
    ]
}
```

3. Add code that attaches the debugger when your plugin launches:
```
import sys
import traceback
from qgis.core import QgsMessageLog, Qgis


MESSAGE_CATEGORY = 'Messages'


def enable_remote_debugging():
    try:
        import ptvsd
        if ptvsd.is_attached():
            QgsMessageLog.logMessage("Remote Debug for Visual Studio is already active", MESSAGE_CATEGORY, Qgis.Info)
            return
        ptvsd.enable_attach(address=('localhost', 5678))
        QgsMessageLog.logMessage("Attached remote Debug for Visual Studio", MESSAGE_CATEGORY, Qgis.Info)
    except Exception as e:
        exc_type, exc_value, exc_traceback = sys.exc_info()
        format_exception = traceback.format_exception(exc_type, exc_value, exc_traceback)
        QgsMessageLog.logMessage(str(e), MESSAGE_CATEGORY, Qgis.Critical)        
        QgsMessageLog.logMessage(repr(format_exception[0]), MESSAGE_CATEGORY, Qgis.Critical)
        QgsMessageLog.logMessage(repr(format_exception[1]), MESSAGE_CATEGORY, Qgis.Critical)
        QgsMessageLog.logMessage(repr(format_exception[2]), MESSAGE_CATEGORY, Qgis.Critical)
 
 class MyQgisPlugin:
  def __init__(self, iface):
    enable_remote_debugging()
    # ... the rest of your plugin code
 ```
 
 4. In VS Code start debugging using the Python: Remote Attach configuration defined above
 
 ## Debugging tasks / multithreading
 In order to line-by-line debug code started in another thread, e.g. when running a QTask, add `ptvsd.debug_this_thread()` after the thread split. E.g. in the `run()` function of your task (here from a function):
 ```
 def run(task):
    import ptvsd
    ptvsd.debug_this_thread()
 ```
 
 ## Acknowledgement
 This gist is based on one written by [AsgerPeterson](https://gist.github.com/AsgerPetersen/9ea79ae4139f4977c31dd6ede2297f90).
 Instead of installing a third-party Qgis plugin for attaching the debugger, I added the code in point 3.