Glamorgan’s start was mostly solid rather than spectacular with the outfield slow from earlier rain, but Lauren Parfitt and Bethan Ellis picked off any poor deliveries to rack up 66 before skipper Parfitt was bowled by left-arm seamer Rachel Slater for 29 off 26 in the 10th over.
Somerset teenager Niamh Holland threatened briefly before being caught behind by Maddie Ward off Olivia Thomas for 13 to start a Glamorgan slide.
Bethan Ellis hammered a straight drive back to Claudie Cooper and Bethan Gammon (11) was caught behind off Ria Fackrell as all Ward’s bowling changes paid dividends.
Poppy Tulloch (14) was the only other Glamorgan batter in double figures, providing Winfield-Hill with her third catch and former England all-rounder Beth Langston with her second wicket.
Winfield-Hill dominated the strike and the scoring, showing her class as Yorkshire rattled up 68 without loss in the first half of their innings.
Winfield-Hill drove Poppy Walker (1-21) to Gammon at long-off, while seamer Tulloch sent back Dutch star Sterre Kalis for one and had Boyce caught behind.
But Ward (21 not out) hit successive fours off Walker to ease the pressure and a flurry of boundaries from Campbell saw Yorkshire through to the final against Middlesex.
Glamorgan Women now turn their attention back to the One Day Cup, where they have three wins out of four ahead of hosting Leicestershire at Neath on Saturday 2 August.
Yorkshire, with the same record, entertain Middlesex on the same date.