Peel strips of zest from lemons using a vegetable peeler; reserve lemons for another use. Put zest and vodka in an airtight container; let stand at room temperature at least 2 days (or up to 1 week).
Stir together sugar and the hot water until sugar is dissolved; let cool. Stir sugar mixture into vodka mixture; refrigerate in an airtight container 1 day.
Before serving, pour through a large sieve into a decanter or serving vessel; discard zest. Pour 2 ounces limoncello into each glass. Limoncello can be stored in the freezer in an airtight container up to 3 months.