Although winger Oakes has played for the club before, scoring six tries in 10 appearances as they won the League 1 title last term, the capture of explosive centre Vaivai is a real coup.
The USA international, who played for South Sydney and Penrith Panthers in the NRL, has been a regular for the Robins this term.
Vaivai, 29, has scored three tries in 15 appearances, often appearing on the wing with Tim Sheens’ side.
Oakes, 20, has made seven starts for the Robins in this campaign.
Both come straight into contention for Sunday’s trip to Widnes Vikings as York, up to joint-third in the Championship, bid to maintain their impressive season so far.
Knights head coach James Ford said: “I'm really looking forward to working with Will again; he’s a very strong ball carrier with the potential to make a big impact for us.
"I'm excited to work with Junior too.
“He’s very explosive and can turn momentum in games with pieces of individual class.
“I’m confident he’ll be a very popular player in his time at York.”
Ford added: "I’m so proud of the clubs efforts to sustain our progress and the support we’ve received from our fan base has facilitated this drive."