In the second paragraph of your link, one learns that only certain individuals with particular conditions actually benefit from herd immunity over simple vaccination. We can assume that none of those people are libertarians, so the "valid libertarian opinion" is simply to vaccinate and let others do as they will. The primary victims of the lessened herd immunity that results from less than total vaccination are the people who don't vaccinate. This is a problem that solves itself. Presumably Gary Johnson simply opposes the creation of some sort of Trumpian Vaccination-or-Death Squad.