First and foremost, read you umbrella policy and confirm that it covers your liability with respect to properties other than your primary residence. The insurance agent gave you wise advice about you being listed as an additional insured on the policy covering your vacation home. This policy should cover the person or entity who is liable. In the event of a lawsuit, it is likely that your, your LLC, or both will be named as defendants. If both the LLC and you are named insured, at least, you will not have to expect issues with your insurance company, such as your carrier trying to deny coverage to you personally by arguing that the liability is solely your and not the LLC's, and thus you not being covered. Being named as an additional insured will not create liability for you personally.