I think the other posts are right in suggesting that the customer probably
expected the shot to be chilled. This is also very popular in the UK.
But if the customer did not specify that, and if your bar has no standard
policy on storing spirits in the chiller, then it was hardly your fault it
came back for a top up with soda. It speaks to me of an inexperienced customer
as much as an inexperienced bartender.
What type of Vodka did they order anyway? Was it a premium brand, or some
generic paint stripper type? This may be the root of the problem.
I would definately advise keeping at least a bottle of good quality Vodka in
the chiller, even if you are not serving that many cocktails, it does the
spirit no harm and is a great time saver.
Cheers!