Customizing the vtid parameter 

The vtid config parameter sets a custom vtid for this user. vtid is used by Visitor Data Mart to identify visitors and by default the vtid parameter is provided by the tag's call to wtid.js (if disablecookies=false) or  is a randomly generated ID (if disablecookies=true).

If the current site has the ablity to identify users and you wish to reuse that ID for segmenting users in Visitor Data Mart, then you can pass that custom id to the JavaScript tag using the vtid config parameter.  

Default Value: undefined

JavaScript Example:

window.webtrendsAsyncInit = function () {
	var dcs = new Webtrends.dcs();
	dcs.init( {
		dcsid: "dcs9x99xxxx9xxx9xx9xxxx9x_9x9x",
		vtid: window['mysite_uid']
	});
	dcs.track();
}