If you are using a weed eater string trimmer, you could damage a vinyl fence. It will depend on how you are using the string trimmer and what type of string you are using. If you have a string that is star-shaped, the sharper edges are more likely to cause damage to vinyl. This type of string is more powerful and is designed to cut through harder materials. If you are holding the throttle all the way down, this will also increase the chances of damaging your vinyl fence. If you are careful, however, you should be able to avoid damage.