Joseph, who turns 28 on Thursday, has been with the Vikings since the start of pre-season training in late October and played in their 30-22 Boxing Day defeat by Warrington.
"Since his arrival on a trial basis, Phil has done everything we have asked of him, fitted in well and become a really strong member of our group," said Widnes coach Denis Betts.
Huddersfield-born Joseph made his debut for his hometown club in 2004 and went on to have spells at Swinton, Hull KR, Halifax, Oldham and Bradford, for whom he made six appearances in Super League in 2012.
He said: "I've worked hard to get myself into this position and I'm really pleased to have been rewarded with a deal."
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