I didn't say how much vodka because everyone is different and I thought I'd leave that part up to you.
In a small saucepan, bring water and sugar to a simmer. Cook until sugar dissolves to create a simple syrup. Remove from heat and let cool.
In a large pitcher/jug combine the simple syrup, vodka, orange liquor and lemon juice. Mix well. Add the soda at the end to help retain the carbonation.