The Adelaide Strikers leg-spinner will replace Australian player Sophie Molineux in the UK Twenty20 competition, who has had to withdraw due to injury.
Wellington will line up alongside West Indies captain Stafanie Taylor and fellow Striker Suzie Bates, as the Vipers' third and final international player.
Vipers' Director of Women’s Cricket and Strikers assistant coach Charlotte Edwards welcomed Wellington to the side for her first KSL stint.
“We’re all disappointed that Sophie has had to withdraw from the KSL but we’re very lucky to get a replacement of Amanda’s quality,” Edwards said.
“She’s a hugely talented leg-spinner and will offer something different to our squad this summer.”
Wellington is the second Striker to have newly joined a KSL club, with Lancashire Thunder announcing Tahlia McGrath as an overseas player last month.
The Southern Vipers will begin their KSL campaign at Liverpool as they take on the Thunder on Tuesday 6 August.