Allen has scored 33 tries in 132 games during two separate spells at the Select Security Stadium.
Meanwhile, Butterworth departs Widnes having failed to make an appearances for the team during the current campaign.
"I'd like to wish Dave and Kieran all the best for the future," Vikings coach Denis Betts told the club's official website.
"Dave has been a good servant to the Vikings during his two spells at the club and he leaves here at the end of our campaign with our best wishes and luck for the future.
"Kieran hasn't managed to make an appearance for the club this year. It has been difficult for us to find him game time elsewhere due to the way the system is and he's too old for Under-19's rugby league.
"However, I know Kieran has enjoyed the full-time environment and being around the culture we have here. We believe he's benefitted from that."
Widnes play their final match of the regular season on Friday away to Salford Red Devils before they take part in the playoffs.