Former England Saxons international Horstmann, who was also a member of the IRB World Sevens Series squad in the 2003/04 season, has played over 130 games for Warriors.
The ultra consistent back row forward is held in high esteem by his team-mates and was awarded the Player's Player-of-the-Year at the club's annual awards at the end of the 2005/06 season.
The 28-year-old repeated that feat at the end of the 2006/07 season when he was named Player of the Year and secured a dream double with both the Player of the Year and Player's Player of the Year award at the 2008/09 awards.
Head Coach Hill said: "Kai has impressed me throughout pre-season and is clearly a very strong leader.
"He is always very measured and what he says is well thought out. You know with Kai that he will always remain calm under pressure.
"It is clear he has huge respect from both the forwards and the backs and so will be a real leader across the squad for us.
"I really believe that Kai and his experience will be the perfect foil for Chris Pennell and they will work closely together to take Warriors forward this coming season."
Horstmann added: "I'm delighted to be asked and massively excited, it's certainly something I am looking forward to doing in what is a big season ahead for us.
"It's great that Richard Hill has put that trust in me and I'm looking forward to helping Chris in his role. We're all pleased that he has got the captaincy, he is a great character and really leads by how he plays and trains.
"We have a huge amount of leaders in the squad already and that makes life a lot easier for us, we are very lucky in that respect, we have a huge amount of experience right through the squad.
"It's a great honour to be asked to be vice captain and I'm looking forward to giving my all to make this a hugely successful campaign for the club."