Wash the lemons and dry them well. Zest the lemons with a fine microplane. Get as much of the yellow as you can -- but avoid zesting the white pith. Put the zest in a 1 gallon glass jar that you've washed in hot soapy water and dried before using. Add one bottle of the vodka. Be sure to save the empty bottle. Cover the mouth of the jar with plastic wrap and then screw on the lid. Give the jar a shake and let it sit in a cool, dark place for a minimum of 2 weeks. I let it sit for at least a month or even two months if I want a really intense flavor.
Combine the sugar and water in a saucepan and cook until the sugar has dissolved for 5 to 7 minutes to make a slightly thick simple syrup. Cool the syrup completely before you add it to the lemon/vodka mixture in the glass jar. Then add the second bottle of vodka to the jar filled with lemon pith and vodka, and let this sugar/lemon/vodka mixture rest for another 2 weeks (minimum) to 1-2 months. Hold on to the second empty bottle, too.
When you're ready to use the Limoncello, line a mesh strainer with cheesecloth and dump the lemon/sugar/vodka mixture into the lined strainer that's been placed over a large bowl. Squeeze out as much liquid as you can and discard the pulp. Pour the remaining liquid into the two saved vodka bottles and store the Limoncello in the freezer until you're ready to use it. (You should taste it before pouring into the reserved vodka bottles. Here's your chance to adjust the sweetness if you think you need to add more simple syrup.)
To make the spritzer pour 1 ounce of chilled Limoncello in each Champagne flute. Add enough cold seltzer or chilled Champagne to make the drink as concentrated as you'd like. Garnish with a sprig of fresh rosemary. (You can also finely chop rosemary and mix it with the Limoncello before you add the seltzer or Champagne to give a more pronounced flavor.)