How to run multiple tasks in VS Code on build?

The problem is that "Run Test Task" and "Run Build Task" do not execute all tasks in that specific group. Usually you get a drop down selection so you can choose which task to execute. Since you have specified one of the tasks as default, the selection will be skipped and instead the default task is executed.

You can work around that by adding dependencies. Take the following example:

{
    "version": "2.0.0",
    "tasks": [
        {
            "label": "Echo 1",
            "command": "echo",
            "type": "shell",
            "args": [ "echo1" ],
            "group": {
                "kind": "build",
                "isDefault": true
            },
            "dependsOn":["Echo 2"]
        },
        {
            "label": "Echo 2",
            "type": "shell",
            "command": "echo",
            "args": [ "echo2" ],
            "group": "build"
        }
    ]
}

As Echo 1 depends on Echo 2, Echo 2 will be executed prior to executing Echo 1. Note that the definition is a list, so more than one task can be specified. In that case the tasks are executed in parallel.

In your case adding "dependsOn":["Compile/minify cms.scss"] to your main build task should execute both tasks.

You can use Compound Tasks.

The example below executes "Client Build" and "Server Build" tasks when "Build" task is called.

{
  "version": "2.0.0",
  "tasks": [
    {
      "label": "Client Build",
      "command": "gulp",
      "args": ["build"],
      "options": {
        "cwd": "${workspaceFolder}/client"
      }
    },
    {
      "label": "Server Build",
      "command": "gulp",
      "args": ["build"],
      "options": {
        "cwd": "${workspaceFolder}/server"
      }
    },
    {
      "label": "Build",
      "dependsOn": ["Client Build", "Server Build"]
    }
  ]
}