Dictionary Headword Search
What I usually use to find an L&S entry using Tuft's Perseus Digital Library is the "Dictionary Headword Search" (URLs for this kind of search contain "resolveform?" followed by search-specific information; you can perform a search of this type by going to the "General Search Tools" page and opening the third box down, labeled "Dictionary Entry Lookup"). This finds "vir" without any problems. You can search for an exact form, or for headwords that start with, end with, or contain a certain sequence of letters.
"Word Study Tool" is a different kind of search (that I find less useful)
The page you linked to in the question goes instead to Perseus's "Word Study Tool" (URLs for this kind of search contain "morph?" followed by search-specific information). I don't use that regularly, so I don't have any tips on how to use it (for example, I don't know whether there's a trick to getting it to find the form vir).
Getting information on non-headword forms
As the name implies, this only searches headwords. If you know the lemma form of the word you're interested in (as in this case) you can just look at the entry and see what it has to say. It turns out that the L&S entry for "vir" doesn't contain any mention of a form "vire".
I actually haven't found L&S to be extraordinarily helpful with non-lemma forms. To figure out whether a form is attested, my usual first step is to search the PHI corpus.