To make gin you first need to start with a neutral spirit base. In gin the flavour comes from the Juniper and Botanicals rather than from the spirit itself, so we want something that is much of a blank canvas as possible. In the UK it's illegal to distill your own neutral spirt. Vodka is the most easily available 'neutral' spirit, and we've found the 'vodka' aftertaste to be minimal.
The reason we ask you to add in inexpensive vodka is because any subtle flavours of a good vodka would be overwhelmed by the juniper and botanicals anyway, so there's not much added benefit.