I love fennel, and this recipe could easily exist in fennel week, rather than this week. That said, the lemon balm in this is one of the key ingredients. The flavour of the lemon balm adds much more than lemon would do; it is less overpowering and adds a perfumed citrus that certainly is necessary.
I have found it odd to use fennel bulbs as the flavouring. Most infused drinks use fennel seed rather than the bulb, however using the bulb seems to add a freshness rather than solely the fennel flavour, so I do not disagree with the recipe at all.
However I personally preferred the gin and tonic without the vanilla, partly because of the cost of that amount of vanilla, but also as I do not think it added much of a flavour. This however might simply be a matter of taste. If I was going to add another herb/spice I would add lemon verbena. This contrasts with the lemon balm by adding a sherberty taste to the drink. it makes it a bit less adult and a bit more fun.
That said, this is a wonderful, refreshing recipe and I hope you enjoy it in its fullness! Find the recipe HERE.