title_body: What fruit spirit is in St. Germain?
upvoted_answer: All I've seen anywhere is that the base is eau-de-vie. Eau-de-vie means different things in different places but, in France (where St. Germain hails from) it basically means any distillate. I'd venture to say, since St. Germain refuses to specify, that it's actually a blend of unaged fruit brandys. If this is the case, it almost certainly includes pear (who's distillate is pretty popular in France).... and if you mix grapes and strawberries (both grown in France) you can come close to something approximating lychee (which I doubt they would ship in). The addition of the elderflower would push the grape/strawberry even closer in that lychee direction. This is all just speculation, a whimsy to kick around over a cocktail. 
  
  Realistically, I'd pick up a bottle of high quality syrup (Monin is popular), and use that to replace the liqueur. You want to use less in the cocktail as it's sweeter, but it'll bear the brunt of the work.  You can add a cheap neutral vodka (like Smirnoff) to thin it out if you like. Don't use water to cut it, it'll change the mouthfeel and water down your cocktail. For further experimentation you can use a combination of pear brandy or schnapps (really anything clear and pear flavored) and vodka, I'd start with two parts vodka to one part pear. You'll want to mix the two because otherwise the pear will likely be too strong. Consider garnishing with an orange or lemon twist to replace the citrus in the St Germain. I wouldn't worry too much about the lychee side of things, it seems any options would run pricey. Yeah, your mix will be lacking something but, no matter what you do it won't be quite be the real McCoy.... let us know how it turns out. Cheers.