So, mix Venetian and Piedmontese together and have them influence completely the local variety of Spanish (which was already quite different from the rest of Latin American Spanish). The result could sound like (and be perceived by non-Argentines/non-Uruguayans) a "Portuñol", but would still be recognizably Spanish.
Actually during the early XXth Century, many intellectuals in Argentina wanted to make the Argentinean variety of Spanish a language on its own. The problem was that written language was the same as in Spain.
I think that if Brazil didn't promote immigration, most of those Italians are likely to go to Argentina. That would like double the amount of Italians our country received, accomplish the OP and have some nice butterflies.