The Statewide Super SA Scorpions 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 New Zealander Suzie Bates and West Indies captain Stafanie Taylor, as the Vipers' third and final international player.
Director of Women’s Cricket Charlotte Edwards at the Vipers 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 Scorpion to have joined a KSL club for 2019, 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.