The afternoon had started brightly for the hosts and they took the lead after 19 minutes when Macani went over when the ball was moved wide from a scrum but Bill Leyland’s kick hit the post.
Salford hit back six minutes later when Ryan Brierley burst through a gap before sending skipper Watkins over the line, with Marc Sneyd’s conversion putting the visitors 6-4 up.
Sneyd had the chance to grab another shortly before the half-hour mark but could not hold on to Cross’ pass and the visitors took a two-point advantage into the break.
But it took Salford only three minutes after the restart to score their second try when Cross collected a looping pass to finish acrobatically in the corner and Sneyd stretched the lead to eight points.
Things took another turn for the worse for the Broncos when Will Lovell was sin-binned for a dangerous tackle on Lafai, which left the Salford man needing treatment.
The Red Devils then went down a man themselves when Vuniyayawa was shown a red card for a tip tackle on Marcus Stock.
However, despite losing Vuniyayawa, it was Salford who came closest to adding to the scoreline, but Joe Shorrocks was held up short before Lafai’s late effort was ruled out.
London: Adebiyi, Bassett, Boafo, Butler, Davis, Hughes, Jones, Kennedy, Kershaw, Leyland, Lovell, Macani, Makin.
Interchanges: Meadows, Miloudi, Parata, Stock.
Salford: Atkin, Brierley, Cross, Dixon, Dudson, Foster, Lafai, MacDonald, Mellor, Morgan, Ormondroyd, Partington, Cust.
Interchanges: Ryan, Shorrocks, Sneyd, Vuniyayawa.