“It’s a strong team with a very good manager,” Vydra told BBC Radio Derby.
“They have ambition to be in the Premier League and I come with this as well because I know how good it is to play in the top league in the world.”
Vydra first moved to England on loan from Italian side Udinese at the start of the 2012-13 season.
He had two spells with Watford, and a stint with West Brom in between, before signing for the Hornets in a permanent deal in July 2015.
Following his season with the Royals, Vydra returned to Watford at the the start of this season, making a late substitute appearance against Chelsea in the Premier League and also playing in the League Cup against Gillingham.
The fee for Vydra breaks the record set when Derby paid about £6m for Bradley Johnson when he joined from Norwich City in September 2015.