Vueling doesn't exactly get rave reviews on Skytrax
http://www.airlinequality.com/airline-reviews/vueling-airlines/
but, as you've recognised, they do operate the only direct service from Gatwick to Firenze-Peretola.
Your alternatives to the same destination are with BA (all year round) from London City or with CityJet (also from London City but only seasonal). There are no direct services from Heathrow.
So, depending upon when you want to travel, CityJet might be worth considering:
https://www.cityjet.com/
The next nearest international to Florence is Pisa. BA fly there from Heathrow (all year round) and Gatwick (seasonally only). easyJet also operate to there from Gatwick (throughout the year). The train journey from Pisa Fermata Aeroporto to Firenze Santa Maria Novella takes between 70 and 90 minutes, with a change at Pisa Centrale. There are usually two services per hour and the fare is just €11.10
http://www.trenitalia.com/tcom-en
Bologna Guglielmo Marconi Airport is also a possibility, with year-round services from Gatwick (easyJet) and Heathrow (BA). There's a shuttle bus to the rail station
http://aerobus.bo.it/en
and then a fast, direct train service to Florence, taking around 35 minutes, for €14.90 (with several services per hour). Alternatively there's a direct bus from Bologna Airport to Florence (with 10 journeys in each direction every day), taking 90 minutes:
http://appenninoshuttle.nwk.it/prenotazioneAffiliato.asp