Looks like it's a replacement bus between Eccles and Manchester Oxford Road on Sunday.
You could always make your way to Manchester city centre on the tram or a service bus, I suppose, or picking up the train (to Widnes) in Urmston or Irlam might be an option.
P.S. According to Widnes Vikings' website, tickets for Sunday (if purchased in advance) are £20 (adults), £13 (concessions) and £11 (junior). It's an additional £2 in all categories if bought on the day.
Surprised that Salford City Reds don't better promote away match ticket sales, don't we get a commission on whatever we sell?