VS Code : variables in custom snippet

As the VSCode snippet docs says, it uses TextMate to power its snippets. TextMate uses variables of the form $name and ${name:default}, and luckily supplies $TM_CURRENT_WORD which may be sufficient for your needs. However there is no built in variable to get multiple arguments directly after the snippet name i.e. $arg1 and $arg2 as variables. Thought you could do a similar effect with interpolated shell code, but unfortunately:

The snippet syntax follows the TextMate snippet syntax with the exceptions of 'interpolated shell code' and the use of \u; both are not supported.

^{Emphasis mine}

However for this simple example, the following indexed variable example is probably sufficient.

<!-- $1 start-->
    $0
<!-- $1 end-->

$i gives you a value to fill in, you can go between each one with tabbing. The $0 is where the cursor goes at the end(the end of the snippet by default). Optionally you can do something like:

<!-- ${1: default text} start-->
    $0
<!-- $1 end-->

and it'll start looking like:

<!-- default text start-->

<!-- default text end-->

with both of the defaults selected to edit.

This all put together would look like this together in the snippets.json file:

{
    "se": {
        "scope": "html",
        "prefix": "se",
        "body": [
            "<!-- ${1:default text} start-->",
            "\t$0",
            "<!--$1 end-->"
        ]
    }
}

As @Mark pointed out, if you want it to work for more than just HTML you can use $BLOCK_COMMENT_START and $BLOCK_COMMENT_END which will vary for each language. The snippet would then look like this:

{
    "se": {
        // Leaving scope off will make it a global snippet
        "prefix": "se",
        "body": [
            "$BLOCK_COMMENT_START ${1:default text} start $BLOCK_COMMENT_END",
            "\t$0",
            "$BLOCK_COMMENT_START$1 end $BLOCK_COMMENT_END"
        ]
    }
}