Southall rates the Dutchman among the best keepers in the country.
He told the Western Mail: "I think Vorm is capable of playing at the top, top level. People have made a mistake by looking at his size and not his ability.
"He's a good role model for goalkeepers because he has proven that you don't need to be 6ft 5ins to be any good.
"He's quick, agile, distribution is great and a decent shot stopper.
"Could he play for Man Utd? I think he's built for Man Utd because he has got the temperament."