Contracts for the deal were signed on 30 June - with all transactions expected to be processed by the end of August.
Vitacress is one of the leading growers and marketers of salads in Europe, with a turnover last year of some £81m.
It has grown significantly since 1951, when Hampshire farmer Malcolm Isaac - now a millionaire - started the business with just 0.4ha of watercress beds. Vitacress now farms some 1,200ha of watercress, baby leaf spinach and rocket in England - on growing beds in Hampshire, Dorset, Wiltshire and Kent - along with Portugal and Spain.
The deal sees the firm's management structure intact, with all of its employees retaining their jobs.
A representative for Vitacress said: "This signifies a new phase in the development of Vitacress, allowing for an increase in the scale of its operations, particularly in the Iberian market."
RAR is a Portuguese group based in Porto with an annual turnover of more than EUR800m (£636m).
It operates in a range of industries, including food, packaging, real estate and tourism. Last year, it bought a majority stake in Isle of Wight-based Wight Salads - one of the UK's largest tomato producers.