Harlequins back-rower Grayshon has already spent a month with the Northern Rail Cup winners, but returned to Harlequins last weekend as he was ineligible for the NRC final, which the Vikings won 34-18 against Barrow.
But the former Huddersfield man will now return for the rest of the season as Widnes look to push for a Grand Final place.
Sporting Director Terry O’Connor said: “Jonny’s two appearances for us have shown the type of player he is and I would like to thanks ‘Quins Head Coach Brian McDermott for allowing him to return to the Stobart Stadium, Halton in a deal which will undoubtedly benefit both parties.”
Grayshon appeared in wins over Featherstone and Doncaster.
Meanwhile, centre Craig Hall, who played in the NRC final on Sunday, has returned to Hull.
O’Connor added: “I’d like to thank Richard Agar and James Rule for their help over the last few weeks in what has been a mutually beneficial experience.
“We have had the chance to utilise a Super League player in our squad whilst Craig has had the opportunity to get some much needed game time at a higher level than reserve grade and will no doubt be returning full of confidence following his displays for the Vikings.