The use of vibrato can be notated several different ways in tablature. In formal guitar tab, a series of "squiggles" appears above the tab staff, directly above the note you should apply vibrato to. The bigger the squiggles, the more vibrato should be applied.
In ASCII tab, most often the ~ symbol is used, generally strung together to appear as ~~~.
Although it doesn't appear frequently, sometimes vibrato will be simply notated with a v in ASCII tab.