The gap was just two points at the interval but second-half tries from Jordan Thompson, three from Youngquest and Daryl Clark's second of the afternoon condemned the Vikings to their sixth consecutive defeat.
Widnes manager Denis Betts can take solace in the fact his side were competitive for large spells but their much-maligned defence was exploited several times.
The Vikings were showing signs of indiscipline, with the penalty count racking up early on, and Ben Cross gave away another for obstructing Brett Ferres deep in his own territory. Although Richie Owen was held up on the right, the ball was quickly spread back across the field for Ferres to cross in the left corner.
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However, with their first real opportunity, the visitors levelled when Joe Mellor's short kick to the right was pouched by Paddy Flynn, who outjumped opposite number Youngquest to touch down, with Rhys Hanbury scoring the conversion.
Clark exploited a gap in the Widnes defence from dummy half for his side's second try. Danny Orr added the extras to give the Tigers a 10-6 lead but Widnes scrum-half Hanbury kicked a penalty on the stroke of half-time to narrow the gap to just two points.
Thompson extended Castleford's lead by spinning away from two Widnes defenders before scampering over from 30 metres.
Widnes' situation was worsened when Hanbury's grubber fell to Ferres and he passed to Youngquest, who sprinted the length of the field to touch down, with Craig Huby adding the extras after Orr had left the field due to a back injury.
Youngquest added his second from Ferres' high cut-out pass and registered his hat-trick on the left to give Castleford a 30-8 lead as the Vikings threatened to completely capitulate.
Clark then added his second of the afternoon, bursting over unchallenged, and Josh Griffin added the conversion before Marsh ended Widnes' woeful second half with a flourish after crossing on the right.
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